It’s a special night at the Coffee A Go-Go, for it’s Bobby Drake’s birthday and his girlfriend, Zelda, has organized him a surprise party. Bobby is rather happy and marvels at the huge birthday cake in front of him, which, as Vera Cantor says, Zelda has baked herself. Also attending the party are Vera’s boyfriend, Hank, and two other students of Xavier’s school, Scott and Jean. Just as the teens start eating, Bernard the Poet approaches the table. He apologizes for intruding and offers to compose a few lines of verse for the festivity. Of course his efforts are rewarded with a slice of cake and Bernard wanders off, already beginning to formulate his rhymes.
Zelda asks Bobby if he would want to dance with a lady, to which he answers in a joking manner that he would like to, that is if she knew any. Zelda doesn’t take offense and pulls him to the dancefloor, also calling out to the band that she wants them to play a waltz now; after all, she is paying for the party. Seeing the other couple dance, Vera urges Hank to dance with her too, but he turns her offer down, excusing himself as too clumsy with his oversized feet. Jean and Scott, however, follow the example and enter the dance floor too, both of them lost in their thoughts. Jean worries that Scott might only be polite to dance with her, as she had no date, and Scott fears that Jean is thinking of her college friend, Ted Roberts.
Just then, two more party guests arrive – Warren Worthington and Candy Southern. Warren introduces his childhood friend to the others and Hank and Bobby can’t help but comment on the dark-haired woman’s beauty. Candy gets along well with the others and, having already heard much about their school from Warren, she says she would like to come and visit sometime. Hearing this, Scott wonders whether the Professor would allow any visitors. The tense moment is interrupted by Bernard the Poet, announcing that he is finished with his ode. He gets on stage and starts reciting: “A birthday’s like a comely lass, from shadowed night she does appear. Yet, all the while, you know, alas – she’ll only see you once a year!” Hank and Bobby are surprised, this being the first time they can actually comprehend one of Bernard’s monologues, but Zelda asks them to be quiet, as this was only the first of thirteen stanzas.
Meanwhile, back at the mansion in Westchester, Professor Xavier has completed some vital experiment. His final calculations having been made, Xavier thinks that the time has come for the ultimate test, there being no turning back now. For weeks, he kept the mystery of the heavy oaken door in the basement locked from his students and it is something he feels he has to face alone. Xavier opens the door and wheels inside, where he finds his evil stepbrother, the Juggernaut, chained to a chair amidst some lab equipment. Cain Marko is still comatose from his last encounter with the X-Men but Xavier says that he won’t be for very much longer. Deep down, Xavier feels that he somehow failed Cain Marko and he thinks back to a fateful unforgettable day in Korea...
(flashback, about 10 years ago)
Under the stress of heavy mortar fire, Cain Marko deserted from the army and fled into a dark cave, Charles Xavier following him. Inside, they found the sacred lost temple of Cyttorak, a glowing gemstone resting on the altar in its middle. Xavier’s warning not to touch it came too late. In the instant that Cain Marko’s hand made contact with the gem, he began to change. The cave was bathed in scarlet light, just as Marko read the stone’s inscription about whoever touching the gem being transformed into a human Juggernaut. Just then, an enemy shell struck a vital spot in the rock above the brothers and there was a cave-in. Xavier managed to get out in time, while Cain was buried alive.
(flashback, several months ago)
Having dug himself out over the course of an entire decade, the Juggernaut had come after Xavier and attacked him and the X-Men. Only the combined might of the X-Men and the Human Torch was able to defeat him, the crucial event of the battle being the Angel robbing the Juggernaut of his helmet, thus rendering him vulnerable to a telepathic attack from Xavier.
Xavier remembers that he turned Cain Marko over to the authorities for treatment after that, but he has remained in a coma ever since. So, he eventually obtained custody of his stepbrother to try and help him by himself. And with that, the Professor prepares to finally do so, hoping to destroy the Juggernaut and restore Cain Marko.
At the Coffee A Go-Go, Bernard has finished his piece and Zelda applauds him. Suddenly, the teenagers hear a load roaring sound and turn their heads. Jean and Scott are the first to see where the noise originate from – a gang of five bikers, heading straight towards the window. They burst through, right into the café, and start circling around the table with the birthday guests. Zelda notices the five guys; they are called Satan’s Saints, their leader being Rocky Rhodes. He had asked Zelda on a date the week before but she turned him down. Obviously, Rocky isn‘t a good loser.
The mutants want to do something about the bikers but they know they can’t use their powers without revealing the secret identities. As the bikers begin to get closer, the girls are getting scared and the men know they have to act. Bobby pretends to slip and, while under the table, he uses his ice power on the front wheel of Rocky’s bike. At his next turn, he skids and eventually slams into a wall of the café, knocked out cold. This, however, only further agitates the rest of the Satan’s Saints and one of them heads straight for the party guests. Candy warns Warren to get out of the way, not knowing that he and Scott have already came up with a plan. On the count of three, they knock over the table and hold it in place as some sort of ramp. The biker has no time to react and drives up the table. His bike out of control, he lets go of it, only to end up in the party decoration at the ceiling.
Jean, meanwhile, has no problem using her power, as it isn’t visible. Telekinetically, she makes another biker’s helmet slips over his eyes. Driving blind, he crashes into another member of the gang, two more now dealt with. That leaves only a single member of the Satan’s Saints unaccounted for and, seeing how his friends ended up, he decides to cut out. He heads towards the crashed window, where Hank McCoy is standing to block his escape route. Hank leaps up, grabs the window frame and swings his feet right at the biker, knocking him out. Shortly afterwards, the police arrive to collect the five goons. Bobby feels sorry for Zelda that her party was ruined but she is just happy that nobody got hurt. Vera and Candy are rather impressed by their men’s bravery.
Back in the secret lab at the mansion, Professor Xavier has finished preparing his Energo-Transformer, a device he created by both employing his own scientific knowledge and some researched arcane studies of the legendary demon Cyttorak. The machine is supposed to drain the Juggernaut of his awesome power and send it back to the nameless void between dimensions where Cyttorak resides. With the power gone, he should be able to cure the embittered mind of Cain Marko. Xavier places himself in a second seat, back to back with Cain Marko, and lowers two helmets, which are connected to the machine, for each of them. After he activates the switch, the machine begins to operate, electricity crackling in the air. Xavier feels that it’s working, his mental energy flowing through the device and meeting the savage hostility of Cain Marko. Carefully measuring the energy output, the telepath knows that he is about to reach a critical point.
Then, suddenly, it goes wrong. Xavier senses some other powerful, alien presence entering Cain’s mind, adding to his own natural resistance. Unfortunately, the machine was only designed to handle two mental forces, not three, and, before Xavier can switch it off, the machine overheats and explodes. After the smoke clears, revealing an unconscious Xavier among the remains of his device, there is one figure standing, exclaiming that he is free – the Juggernaut. Seeing his stepbrother in front of him, Cain Marko has no idea what happened, not that it really matters to him. He picks up his helmet, which Xavier had stored nearby, and, once it is in place, the Juggernaut is once more surrounded by an aura of sheer energy. Thus reenergized, Cain fells yet another ability coursing through him – allowing him to mentally probe the building. The Juggernaut realizes that he must have absorbed his stepbrother’s mental powers. Not wanting to give it back, he thinks that he first needs to dispose of Xavier’s students, who, as he mentally senses, are currently not present.
In New York, Bobby is seeing Zelda home to her flat. Leaning in the doorway, Zelda once more apologizes for the party having turned into a disaster and wonders if there was some way she could make up for it. Bobby reassures her that he really enjoyed himself but, now that she mentions it, there is indeed a little thing she could do for him, as he leans in for a kiss. A few minutes later, Bobby joins Hank, who was waiting downstairs after seeing Vera home, and Hank can’t help but notice his friend’s ecstatic manner.
Warren and Candy have made it back to her apartment at the Upper East-Side. Warren is impressed by the place and thanks her for inviting him in for a drink. However, he politely says that it’s getting late and puts on his coat, ready to leave. On his way out, he tells Candy that he is looking forward to seeing her soon and asks her for a date on the next Saturday. Unfortunately, she says, she’s got already an appointment for then but she would like to meet at some other time. Warren doesn’t take it too hard, answering, “If at first you don’t succeed... cry a lot.” However, by the time he reaches his car, Warren thinks that he is really attracted to candy and that they have liked each other ever since they were kids. But he doesn’t want to be unfair to her and, before getting carried away, he knows he needs to get over his mental rebound from Jean.
The lovely redhead, meanwhile, is in a midway park, along with Scott Summers. She comments on the nice weather, though actually she wouldn’t care whether it rained or stormed, if only she can be near to him. Likewise, Scott, who says that its getting late and they have to get back to the school, thinks that it can’t go on like this and he gathers all his strength to finally talk to the woman he loves. He starts, saying that surely she and the others must have wondered why he is sometimes rather aloof and some sort of a loner. Scott explains that this is due to him thinking that his deadly eyes set him apart from the rest of the world and that he fears letting anyone too close, for he might accidentally harm them.
Jean reveals that she suspected as much but tells him there is no need to feel so totally alone; there are people in the world who can relate to that and care about him. Scott thanks her for listening. It’s good to know that she understands him. As the two mutants head back to Westchester, Jean thinks to herself that now she has found an opening and she won’t rest until she breaks through the rest of Scott’s shell to find out whether he really loves her or if he is only looking for pity. Scott, meanwhile, is relieved, their conversation convinced him that Jean has feelings for him and he knows that he must tell her that he loves her soon.
As Warren’s car returns to the mansion in Westchester, his arrival is spotted by the Juggernaut from a window. Warren’s suspicion is aroused, as the entire mansion is dark and there being no sign of the Professor, so he changes into costume before he enters. It all reminds him of how, not too long ago, he walked into a trap by Magneto. Warren takes to the air and flies through the buildings’ corridors checking on each room but finding no trace of Xavier. Finally, there is only one place left to check – the basement, and Warren wonders whether he could take a look, despite Xavier having declared area off-limits for the students. Just as Warren is about to open the door, he hears something moving behind him. The Angel dodges in time to evade a mighty blow from someone’s feet.
The lights are turned on and Warren realizes that his attacker was the Beast, the two X-Men not having recognized each other in the dark. They are joined by the rest of the team, all wearing their costumes too, as they had the same mutual feeling of danger when returning home. Even worse, Cyclops says that he was unable to contact the Professor with his signal-watch. Before they can continue their search a loud alarm goes off, caused by Cerebro. The X-Men know that it must mean a dangerous opponent is near and quickly check out the control panel to learn who it is. However, it’s their own five names they see displayed. Then, without warning, something shakes the entire building, toppling over Cerebro and smashing it beyond repair. Robbed of their only means to locate Xavier or the villain they are up against, the X-Men pinpoint the source of the vibration to directly below, deducing that whoever they are up against must be in the Professor’s secret lab in the basement.
With caution, the five mutants enter the stairway downward, weary of being attacked at any time. It’s no use, though, as a voice from a dark corner of the basement tells them, and, activating a certain switch, the stairs turn into a sliding ramp. One of the mansion’s defensive devices turned against them, the five mutants slide down, only Angel and Marvel Girl can stop their decent by using their abilities. Warren wants to buy the others time to regroup and heads towards the corner where the villain was talking from, despite Cyclops’ warnings of them yet having to learn the foe’s identity. The hotheaded Angel doesn’t care, having spotted something move behind a stack of crates. Just as he almost reached them, the crates suddenly fly straight at him, knocking Warren out.
The other four X-Men look in disbelief, as the Juggernaut stands revealed. After checking on the Angel, Cyclops demands to know what happened to Xavier, prompting the Juggernaut to gloat that soon they all will be united in death. However, he says, he didn’t lure the X-Men down there to talk but to have vengeance for the long months he spent in the coma. The X-Men spring into action, Cyclops ordering them to use Plan F-1. The Beast bounces of the walls to get on the right side of Cain Marko, while Iceman slides around his ice-ramps to get to the villains other side. Jean is somewhat worried, noting that the Juggernaut seems even more confident and powerful than when they last fought him but then she too gets into action, attempting to loosen the screws of the villain’s helmet with her telekinesis.
It doesn’t work. Cain Marko sees through the X-Men’s distractions and, just as his invisible force field blocks Marvel Girl’s telekinetic attempts, he creates enough raw energy to send the leaping Beast careening into Iceman, knocking out both of them. Just as he claims to be superior to “mere” mutants, Jean gets desperate, fearing there being no way to stop him. Cyclops stays calmer than her, though his only thoughts are to get Jean to safety. Shooting his optic beam at the Juggernaut, he tells the woman he loves to escape and warn others of the threat of the Juggernaut. Jean does as told, though not to save herself but intending to fetch some weapon from the mansion’s armory in hope of defeating the Juggernaut. She tells herself to hurry, as she tries levitating up the ramp.
Scott fires his beam at full force but the Juggernaut still gets closer to him, step by step. At least, he thinks, Jean will be safe but the Juggernaut answers this stray thought, saying that she won’t. Hearing this, Cyclops realizes that the Juggernaut must somehow be able to read his thoughts. The Juggernaut confirms this suspicion, though also demonstrates that, for stopping the redhead’s escape, he doesn’t need mental powers. He slams his fist on the ground and the resulting shockwave makes Jean lose concentration, cancelling out her telekinesis. Scott, who was closer to blow, already unconscious, she finds herself the only one left standing. Regaining her courage, Jean says that as long as one X-Men is left standing, he hasn’t won. She once more tries to fight but finds herself unable to deal with a mental blast from the Professor’s stepbrother. Within moments, she too slumps to the floor.
The Juggernaut stands triumphant and, raising his arms in victory, he aims for the crowing touch – he wants to bury his opponents in their own mansion. Just as he starts smashing up the walls, he hears a voice in his brain. It tells him to forget about Xavier, as there are more important matters right now. The voice introduces themselves as Factor Three and they offer to ally up with the Juggernaut. In fact, it was they who caused the explosion that freed him from his coma. His curiosity peeked, the Juggernaut wants to learn more but Factor Three tells him they will talk later, after he has stolen a transatlantic jet and joined them at their secret base in Europe. After exclaiming that he won’t be commanded and is only joining them because he wishes to do so, Cain Marko smashes himself a way out of the mansion and departs.
In his wake, he fails to notice a broken water pipe, slowly flooding the damaged basement. The water revives the unconscious Hank McCoy, who soon wakes his teammates. Quickly, the five mutants locate the fallen form of their mentor, who is still breathing but won’t wake up. Jean finds the switch that turns the ramp back into stairs. A short time later, in the mansion’s infirmary, the X-Men find their worst fears coming true. Xavier is comatose, apparently clinging to life only through sheer will power. They can do nothing to help him and, with the Juggernaut still on the loose, they know that Xavier would want them not to bother with him but capture the villain instead.
Making his way towards New York City, the Juggernaut thinks that he cares nothing about Factor Three or for whatever purpose they freed him. However, if they are as powerful as he suspects, they might be worthy allies for world domination.