A subway train speeds through the labyrinthine tunnels underneath New York City, when suddenly a strange creature appears on the tracks. Several feet taller than the average human, he has pale yellow skin, grayish, fur-like hair and rage-driven red eyes. He’s clad in metallic armor and raises his fists, ready to attack. As soon as the trainman becomes aware of someone standing on the tracks, he tries pulls the breaks, but the train is riding on top speed. The strange bulky creature apparently has never seen a subway train before, considering that he thinks it to be a monster trying to attack him. He slams himself into the train, bringing it to an immediate halt and knocking it off the tracks.
Riding the besieged train are two students of Xavier’s school in Westchester, Hank McCoy and Bobby Drake, along with their respective girlfriends, Vera Cantor and Zelda. Naturally, with the train stopping rather abruptly, the four teenagers wonder what’s happening up ahead and, although Hank and Bobby want to investigate, they know they can’t do anything without giving away their secret identities. However, the lights suddenly go down, thus providing the two X-Men with the perfect opportunity to sneak out of the train and change into their costumes. Because of Xavier having taught them to move in the dark, they have no problems to make their way through the train.
As soon as Iceman and the Beast reach the front of the train, the strange creature immediately attacks them saying "Keep back! None may approach me...and live!" Beast tries to knock out the creature by leaping feet first, aiming for his head, but is hurled back. During his attack, Hank referred to his opponent’s physique as “grotesque” and the creature decides that “Grotesk” will be his name from now on, for that which he once was he is no more.
Trying to help his teammate, Iceman throws an ice-boomerang at the creature since ice was also effective against Frankenstein's monster, but it has no effect on this creature. Next, Iceman entirely covers Grotesk in ice, but the Beast warns him that this is not Frankenstein’s monster, and as such he might not be affected the same way. As if to prove Hank correct, Grotesk easily breaks free, with ice-fragments being sent flying in all directions.
The two X-Men plan their next move, however Grotesk seems not interested in fighting anymore. He turns around and heads for a hole in the tunnel wall, through which he must have originally entered the tunnel. With his loud, booming voice, he warns the two mutants that he has no time to waste in useless combat, but that they should beware if they meet again. He then seals off the hole behind him with a large boulder, and Beast finds it too heavy for him to move. As they see lights approaching from the subway station ahead, probably the subway police who must have heard the noise from the fight, Hank and Bobby decide to head back for the train and change into their civilian clothes.
A short time later, Hank, Bobby and their dates are walking through New York’s Greenwich Village. Zelda demands an explanation for what happened to Hank and Bobby when the lights went out, and Bobby says he did what “any red-blooded, all-American boy would do when he’s alone with the love of his life! namely, I went lookin’ for a match!”
Vera is also not pleased with how the evening turned out, after not hearing from Hank for weeks, now he suddenly announced that he is cutting their date short. Hank says he’s sorry but something rather urgent came up, forcing him and Bobby to return to the school. He thinks to himself that Xavier will want to now about the strange being from the subway tunnel without delay. Zelda says that she and Vera have no other choice but to believe them, but she warns Bobby not to be two-timing her.
Meanwhile, Grotesk wanders through some dark caverns beneath the city, trying to remember where he came from and who he is. For now, he considers the name Grotesk fitting, but he recalls that he was not always like this. Grotesk once more strains himself to remember, and indeed some bits of his memory return...
(flashbacks, over the course of the last few years)
There was a civilization living in the endless caverns below the surface, most of them living in a great, striving city called Metropolis. Unsuspected by humanity above, the subterran people had been at war with each other for years, trying to eradicate each other and gain dominance over the other races. Grotesk’s people were as oblivious to the existence of humanity, just like the humans didn’t know about their underground activities either, only every once on a while they witnessed their effects - like when the subterran wars burst the very mountains asunder, creating volcanoes.
One day, in the teeming area that was the center of their subterran empire, King Krono announced that the time had come to explore the outer world and learn if there were other races that they could conquer and enslave. He never got to finish his speech, though, for at precisely that moment a large explosion destroyed the subterran empire. The world as they knew it came crashing down around them, and Krono’s son, Gor-Tok, tried his best to calm down the subterrans who were running around in panic. He rallied his followers into standing their ground against whoever was attacking them, but the people that it was no use against such massive force.
When the violent trembling stopped, Gor-Tok once more called out to the few of his subjects that had survived, and swore that they would get revenge on whomever was responsible for this attack. He reminded them of the many other cities and the numerous members of their race that would join them in the attack. However, in the wake of the mighty blast came a strange illness, which spread through the entire subterran empire, killing them all. One of the last to succumb to it was Gor-Tok’s wife, the lovely princess Ingar.
Grotesk now fully remembers his prior life as Prince Gor-Tok, and he also knows that those fear-filled days were brought upon his race by humanity. Wandering through the caves underneath New York, he wonders what good this knowledge does him, though, for he is all alone, and he shall ever remain that way. He was changed by the radiation, changed - almost as if fate itself decreed that one of his race should survive to wreak an awesome revenge on humanity. His face becomes grim with anger again, as he declares that he, who was once far more, is now less than human. As such, humanity shall know him as Grotesk, the Sub-Human. It was the humans atomic radiation that turned him into what he is today, that killed his father, his princess and his entire race, and for that he will destroy the outer world even if it is the last thing he will do.
At Xavier’s school, despite the late hour, there’s a training session in the Danger Room. First it’s Cyclops’ turn, whom Xavier wants to narrow his optic beam at its most narrow, to nudge open the lock on a door. Scott does as told, but at the same time he thinks about the professor seeming tense and short-tempered today. He wonders if it’s the strain he has been under recently, or something else. Thus distracted, Cyclops uses too much force and smashes the lock, instead of just nudging it.
Next, Xavier turns to Angel, saying that his test in evasion tactics is up next. As an exercise in aerial maneuverability, he orders Warren to fly with his hands by his sides, and the winged X-Men does rather well evading several force beams while flying at top speed. Warren hopes that his good performance will make up for some of the professor’s anger about Cyclops, when suddenly he is caught by surprise when he passes by a suction duct. The air stream is pulling him in, and Xavier orders Marvel Girl to rescue him with her telekinesis.
Jean does her best, all the while the professor yells at her “Faster... Faster!” reminding Jean that, in his present state, the Angel is helpless and would be at the mercy of the team’s enemies. Eventually, the young redhead manages to pull Warren free, at which point Xavier ends the session. He reprimands the three X-Men for their poor performance. It took Marvel Girl eight seconds to rescue Angel, and in that time they would have been both destroyed by an evil mutant. Scott interrupts, asking the professor if he isn’t a bit rough on them - after all he can’t expect perfection. However, that’s exactly what Xavier asks for - perfection. Anything less in their sworn roles as heroes is virtual suicide.
At that point, Hank and Bobby arrive back at the mansion and enter the Danger Room. However, before they can even begin to tell the professor of their strange encounter in the subway tunnel, Xavier yells at them for entering the Danger Room in their civilian clothes. Hank tries to downplay their mistake as a “trifling incidental,” but the professor gets even more angry about it, saying there is no such thing when one is a member of the X-Men. He concludes with a statement that rules are made to be obeyed, and that a deviation from them might spell disaster. Hank apologizes, which the professor accepts, though he also give Hank and Bobby two demerits and no permission to make any trips into the city for the next month. He then leaves the room, saying that he has other matters to attend too, and interrupts Hank, when he tries to speak again, saying that he has already said enough.
Anything else they have to say must wait until tomorrow, Xavier says as he leaves the Danger Room, though not before asking Jean to accompany him as he wants to talk to her in private. She follows him right away, thinking to herself that only she can even guess part of the reason why the professor is so cold and stern tonight. Once they are in Xavier’s office, Xavier admits to Jean that he got a bit carried away, but he thinks it essential for the X-Men to become a perfect team - now more than ever. She agrees, though Jean also wishes he would tell the others what he recently told her. The professor doesn’t want to hear about that, though, saying that she needs to trust his judgement. He then asks her if she is ready to continue their experiments, which she confirms.
In the Danger Room, the four male X-Men are still taken aback by the professor’s attitude. Scott asks Hank what it is that he wanted to say to Xavier, and thus reminded, Beast fills him and Angel in on their encounter with Grotesk.
Meanwhile, at Archer College, in the heart of Manhattan, one Dr. Hunt is filling in some members of the college board on his latest invention. It’s a device called 'Nuclear Oscillotron' and capable of creating Earth tremors. Hunt’s audience is rather skeptical, and they demand a minor demonstration if Dr. Hunt wants them to allow him to further discuss his far-fetched theories in his classes. Dr. Hunt thinks that it is fair enough and, a few seconds after activating his machine, there is a minor earthquake.
In a cavern far below the city, Grotesk also feels the shockwaves and curses at yet another Earth tremor. Actually, he came to this of all of his people’s areas, because in that region there were never any earthquakes. Not only is it an unnatural phenomena, but it also serves as a painful reminder to the vibrations of the explosion that killed a large number of his race back then, and of his own helplessness so far in getting even with humanity. Grotesk knows that he doesn’t really do any harm by attacking some of their vehicles.
With yet another tremor hitting the cavern, Grotesk is now entirely convinced that they are no coincidence, but that they must have been directed at him by the outside world. He vows to find the cause of these tremors, and to smash that cause and its creator. As he climbs upwards, though, Grotesk gains another idea. Perhaps he might use the surface dweller’s device, whatever it may be, to destroy Earth itself and its population – even though it would mean his own death too.
Back at Archer College, board member Mr. Chalmers tells Dr. Hunt that he has proved his point and that they are convinced the nuclear oscillotron actually works. However, he fears that he may have heralded the end of the Earth. Dr. Hunt calls that rubbish, but Mr. Chalmers asks if he is really sure that one minor Earth tremor created by his machine couldn’t accidentally start a dreadful chain reaction that might tear the planet apart. Dr. Hunt thinks that this idea is ridiculous; he intends for his invention to be used for the benefit of mankind – in ways they can not dream of yet. Chalmers replies that they’ll discuss it later, but for now he suggests to have the machine moved to another location where it can be closely guarded. At least to that Dr. Hunt agrees.
In Westchester, Professor Xavier and Jean leave the office and find the other X-Men still taking about what Hank and Bobby experienced earlier. Bobby is concerned that they are up past lights out time, but Xavier reassures him not to worry, as he has more urgent things on his mind. Bobby is surprised that Xavier knew his concerns, so the professor reminds him that he is capable of reading minds, after all. The telepath then orders his students to return to the spot where Bobby and Hank first encountered the strange being, all but Marvel Girl. While Hank and Bobby hurry to change into their costumes, Cyclops wonders about Xavier’s sudden interest in Jean. He actually wonders if he could secretly be in love with her.
Soon afterwards, the four X-Men have reached the subway tunnel, where the large boulder is still blocking the hole in the wall. Cyclops tells the Beast that he needs to loosen it a bit so that he can do the rest with his eyebeams. Hank gives his all while pushing and, although he didn’t think he could manage it, the rock moves slightly. Now, Scott blasts at the rock, clearing the way without spending all his optic power. However, before Cyclops can enter the cave behind the hole, Hank leapfrogs over him, thus ensuring that he is the first inside. He wants to make sure that, if Grotesk is waiting, it’ll be he who confronts him first. However, there being no sign of the pale-skinned monster, Angel flies into the cavern, and starts doing reconnaissance.
Iceman, Beast and Cyclops venture deeper into the cave and are amazed by the ruins of the ancient civilization presenting themselves before them. They realize that not only it extends as far as the eye can see, and even further, it’s been abandoned for several years. In a huge chamber, they find Warren again, saying he has spotted no trace of Grotesk or anyone else in the area he has searched so far, which would be every inch up to a point where the cave is sealed off again. However, the Beast’s sense of smell tells him that Grotesk was recently there. Cyclops decides that they tell Xavier that they have found Grotesk’s lair, however Warren has already tried to contact him no success, as their wrist-radios do not function because of their being too much radioactivity.
Hank wonders why the professor doesn’t contact them with his telepathy, and Angel answers that it might be blocked out somehow, or maybe he is just too busy with some other matter. Cyclops reaches a decision, in either case they have to get in contact with their mentor, which is why he sends Iceman and Angel back to the school, while he and the Beast will wait in the cavern. Iceman is not too happy with that idea, but Scott tells him that it’s an order.
By now, Grotesk has arrived at the lab complex Archer College and searches for the device that was creating the tremors. He runs into a lab assistant of Dr. Hunt and demands from him the location of the device, but at the sight of Grotesk, the poor man faints. To Grotesk, this is once more proof that the surface dwellers are a race of cowards; then again, he thinks they are right to tremble in fright before his power. He picks up the unconscious man and drags him around, still intent to find the weapon with which he wants to obliterate the planet.
Some time later, Iceman and Angel reach Xavier’s mansion again, and find that Marvel Girl is still awake. When asking her where the professor is, she answers that he got the X-Men’s call and left before Warren and Bobby returned. She offers them to brief her instead and, once they have finished their report, Bobby and Warren assume that she will join them in the subway cavern. However, she declines, saying that she can’t accompany them, which rather annoys Angel.
He outright accuses Jean of lying about Xavier’s whereabouts. Turning her head away from him, Jean tells Warren to think what he wants, but she still can’t go with them. She asks her teammates to believe her that she is terribly worried about their safety, through. Angel notes that she acts somewhat distracted, as if her mind is elsewhere. It is almost as if she and the professor no longer care what happens to the X-Men or to mankind.
Not having found the weapon in the lab, Grotesk returned to his underground base, though he has taken the unconscious assistant along. When he comes around, the assistant at first thinks he had a crazy dream but, seeing Grotesk’s face again, he knows it was no nightmare. The Sub-Human grabs the assistant by his coat and demands for him to reveal where the weapon is to be found, or else he will crush him with his bare hands. Grotesk doesn’t believe the man when he says he has no idea, and is ready to start beating him up, when a voice tells him to put him down.
Grotesk turns around and sees the Beast and Cyclops. The former he recognizes as one of the pair he earlier fought, but to him it makes no difference if its one, two or a hundred opponents he has to face. No matter what, he’ll destroy them. Cyclops and the Beast prepare themselves for the fight, they know they’ve got to hold Grotesk off until the rest of the X-Men return.
The confused Scott Summers has reluctantly accompanied the evil mutant Jack O’ Diamonds to a nuclear power plant, where they are confronted by Professor Xavier. The villain is ready to attack the telepath, but Xavier is able to hold him at bay with his mental bolts. But that’s the only thing he is capable of; Jack’s mental defenses are strong enough for the professor to take over his mind.
Jack orders Scott to fire his eye beams at Xavier, now that his mental energies are focused on him, but the troubled teenager does not obey, exclaiming that, no matter what, he won’t become a cold-blooded killer. Jack Winters knows that the professor’s telepathic powers are stronger than his own and sooner or later he would lose their mental duel, so he decides to use his other abilities instead. With his diamond fists, he smashes through a steel support of the building and hurls it at Xavier, who barely evades it.
The heavy girder smashes into a nearby wall instead, causing the ceiling to collapse. To not be buried alive, Jack Winters rushes Scott outside. When the teenager asks about the man in the wheelchair, Winters replies that they can only save themselves. Moments later, as swirls of smoke rise from tons of fallen debris, Winters tells his unwilling sidekick that he has plans for Scott’s optic beams, and that they start now. Hearing this, Scott understands why Winters was worried about him getting out of the collapsing building in time - because he needs him to break into the Cyclotron center. Jack O’ Diamonds confirms Scott’s suspicion; he thinks that taking an atomic bath in that device will turn his entire body into bullet-proof diamond. Scott once more asks about the bald man in the wheelchair they left behind, but Winters says that he only got what he deserved for interfering.
A few minutes later, the two mutants have crossed the distance to the building housing the Cyclotron. Along the way, Scott thinks to himself that Xavier’s mental blasts must have weakened Jack’s limited teleportation powers, or else they wouldn’t have to run across the estate. However, Scott also knows that the villain is still as dangerous as before, and decides to play along for a while. As they reached the entrance, they are spotted by a guard, who draws his gun and tells the pair to stop. Jack Winters knew they would run into a guard sooner or later, and wants to finish the guard off with his diamond fists, but Scott is faster, smashing the man’s gun with an optic beam.
While the frightened guard runs away, Winters congratulates Scott on his quick move and storms into the building. The villain is so eager to get to the cyclotron that he doesn’t notice Scott staying outside and hiding in a grove of trees, trying to figure out what to do next. He only used his optic ray, because otherwise, Winters would surely have killed the guard. Scott hopes that, without his help, Jack won’t get to the cyclotron chamber, but a crashing noise from inside the building tells him otherwise.
Inside, Jack has reached the mammoth structure, which is the very nerve center of the nuclear complex. He knows time is of the essence, as the guard will surely call in some reinforcements, and he curses at himself for not killing him off when he had the chance. Winters then reaches the console from which the cyclotron is operated, and talks to himself that this is what he needed Scott really for. He would have felt better with someone actually operating the cyclotron while he gives himself a full dose, but he can also program them to work automatically.
As he enters the cyclotron chamber, Jack Winters is bathed in radiation, and feels that it is working. Soon he declares, the radiation will transform him into a living, indestructible diamond, and then no power on Earth will be able to stop him.
Elsewhere on the compound, against Jack O’ Diamonds’ assumption, Professor Xavier is still alive. He intends to find the villain and young Scott Summers before it is too late and, wheeling across the compound, he spots a jeep carrying three guards. Xavier overhears that they intend to stop two mutants from damaging the cyclotron, so he knows that this is where he will find Winters and Summers. Indeed he does, for as the armed and combat-ready guards approach the area where Scott is hiding, he releases another optic beam. The confused teenager thinks that he should probably give up and surrender, but he knows that the guards would then go up against Winters, which they probably would not survive. The guards jump out of their jeep and run for cover, ready to use their guns against the dangerous mutant hiding behind the trees.
Hearing this, Xavier decides he has to act. Hoping that his mental powers have rested enough from their previous use, he telepathically immobilizes the guards, putting them in a state of virtual suspended animation. It works, and all three collapse before they could open fire. Nearby, Scott is even more confused than before, and wonders if this is some sort of unknown side-effect of his optic blast, when he suddenly receives a mental message from Xavier. He introduces himself and explains the he caused the scene. Scott is glad to learn of the professor still being alive, and agrees to help him stop the villain. Although he has no real idea of who Xavier is, Scott somehow knows that he can trust the bald man in the wheelchair.
Inside, Jack Winters has heard the noise of Scott blasting at the guards’ jeep, and decides to speed up the process a bit. He flips down a lever, increasing the power flow of the cyclotron, which now bathes his body in iridescent red shimmering energy. “More power! More! More!” he exclaims, until suddenly the light and humming of the device cease. Stepping out of the dark chamber, Jack Winters feels different - heavier somehow. He decides that he has to leave as fast as possible, which would be through the wall next to him.
Winters walks forwards, and the brick wall tears open as if it was a wet paper bag. As the moonlight shimmers on his now entirely diamond body, the villain declares that nothing and nobody will stand in the way of the Living Diamond!