X-Termination

Billions of years ago, the Celestials conducted an experiment... but it went wrong. To save the universe, they copied reality again and again and erected barriers between them. The experiment was cast into the void between realities and trapped there. Recently, however, the barriers have become weak due to the dimension-hopping activities of various superheroes and villains. This has allowed the experiment to break through the cracks and threaten the entire multiverse.

Oblivious that Wolverine and the X-Men are closing in on him, Nightcrawler is in a park in San Francisco, waiting for a meeting with the Dark Beast. In the meantime, he watches the 616 version of his wife Linda, who is happily playing with her family, and becomes depressed over the fate of his own wife. Dark Beast arrives with a pair of modified goggles, which he explains that they allow them to enter the Dreaming Celestial safely in a bid to return home. Kurt teleports them inside, where McCoy attempts to open a portal to their home reality. However, before the portal stabilizes, the machine he was using begins to malfunction. Before he can fix it, the X-Men arrive and the two have to flee, with Nightcrawler teleporting them through the portal. However, once they arrive in their homeworld, the portal doesn't close behind them, which worries the Dark Beast. 

Nightcrawler tells him the X-Men will help, only for McCoy to break the news that pretty much all the X-Men on his world are dead. Kurt teleports away to see the truth for himself and runs into Prophet of the X-terminators, who is investigating the portal that has just opened up in New Apocalypse.

The two teleport back to find the rest of the X-terminators having arrested Dark Beast and are now investigating the portal. Dark Beast tells them he didn't cause it but they assume he is lying. The situation is complicated first by Wolverine's team, which comes through the portal to take Kurt back to their reality, and then by Dazzler and her team of X-treme X-Men, who seem keen to close the portal. Howlett and Cyclops, from the X-treme team, are particularly concerned with something they saw within the portal. As Prophet starts ordering his squad to evacuate part of the city, Xavier of the X-treme X-Men uses his powers to try and close the portal but it's too late and three huge monsters emerge from it. Xavier tries to tell them they mean no harm but one of the monsters kills him and dissolves his body to absorb its energy. As it does so, it grows slightly bigger.

The two teams of X-Men launch an attack on the monsters whilst the X-terminators hold back and watch the scene. It quickly becomes apparent that the monsters absorb all types of energy, such as that from Dazzler's attack, and are barely affected by physical attack, such as Wolverine and Howlett's claws. It's only Northstar, who is able to hit them and escape quickly, who does them any real harm, and Iceman, who manages to temporarily freeze them. Via Karma's brief possession of them and Sage's telepathic probing, the teams learn of the monsters' origins and their ultimate goals. Once free of Iceman's powers, the monsters split up. One enters the portal to the 616 universe, while a second one heads off in the direction of the city of New Apocalypse. The third remains behind and begins to absorb the energy from the portal.

Acting quickly, Northstar flies Howlett, Hercules and Wolverine through the portal to stop the first monster, whilst the rest tackle the two monsters that stayed behind. After much persuasion, Prophet reveals a dangerous option: the power of Apocalypse is contained somewhere in the city and can be used against the monsters. After reluctantly deciding that the only way to take on the monster is by using that power, the teams split up. Dazzler, Cyclops, Iceman and Nightcrawler will stay in New Apocalypse with Jean Grey and Dark Beast, whilst everyone else will head through the portal to the 616 universe. However, the monster feeding off the portal is in the way of their escape, so Sabretooth and Horror Show volunteer to distract it. After saying their goodbyes the two men go off and sacrifice their lives, allowing the others to sneak into the portal.

When Northstar and his group arrive on the other side of the portal, they find the monster feeding off the Dreaming Celestial. The monster barely acknowledges their attempts to stop it until Hercules attacks, however even the Greek god is no match and he is killed. As Kid Nightcrawler arrives and evacuates Wolverine and Howlett, monster returns its attention to the Dreaming Celestial, which actually tries to escape. However, it doesn't get away in time and the monster links onto it again and kills it, absorbing all the energy from it into itself.

On the other side of the portal, the monster in New Apocalypse is causing enormous devastation. Many of the other heroes of the city try and battle it but they are killed and absorbed by it.

Meanwhile, as Jean Grey and her group search the caverns beneath Humanity's Last Stand base for the Apocalypse seed, the Dark Beast gives the group the slip. More concerned about their objective, the group continues and eventually finds the canister in which the Apocalypse seed is kept. However, much to her group's surprise, Jean immediately has Nightcrawler teleport both of them away, explaining that she can only trust him, as she doesn't really know the rest of the group well enough. She also reveals that Prophet's ultimate plan is for her to take on the power herself, as she has the most experience at wielding god-like powers. However, before she can access the seed, Dark Beast suddenly attacks them from the shadows and snatches the canister off them. At that moment, the monster that was attacking New Apocalypse finds them and bursts through the roof of the cavern. Jean grabs the unconscious Nightcrawler and runs away to join up with the rest of the group.

Back in the 616 universe, the monster has grown enormous after devouring the power of the Dreaming Celestial. Far below it, Prophet and Sage find the black box from the Celestial and examine it to try and find any information that could be useful. To their horror, they discover that there are billions of monsters heading their way. Prophet has Kid Nightcrawler teleport himself and Howlett back through the portal to warn Jean, only to discover the Dark Beast trying to access the power of Apocalypse, all the while being tracked by the monster. Dark Beast escapes again with the power again, followed by Jean and Nightcrawler.

Prophet explains to those who remain that more monsters are on their way and the only way to stop this is to close the portal and save one world or the other. Dazzler refuses to accept the plan and orders him to go save Jean whilst she and Sage attempt to lure the monster in the 616 universe back through the portal again by firing all their energy attacks at it. When the monster doesn't go for it and continues siphon energy from the environment around it, Kid Nightcrawler decides there is only one way to get it back to New Apocalypse and so teleports it through the portal. Already weakened by the numerous jumps through the portal, he dies in Dazzler's arms.

Elsewhere, Prophet, Nightcrawler and Howlett corner and fight Dark Beast in a bid to get the canister off him, but it's ultimately Dazzler who takes him down in the end. She hands over the canister to Jean, who accesses the power and is transformed by it. Though she still manages to hold onto her memories and personality, the Apocalypse power slowly starts to corrupt her. She locates the two monsters, which are attempting to create another portal from which to drain its energy, but when Jean attacks them, they turn their attention to her. Powered by the Apocalypse seed, Jean manages to hold her own against them both.

Meanwhile, Nightcrawler feels guilty about causing the whole situation and, after having seen Kid Nightcrawler die to save the world, he regains a sense of worth again. Ordered by Prophet, Nightcrawler teleports has him teleport through the portal to fetch Sage and the black box, but he finds it harder to travel through the portal as pieces of the barriers of reality clutter it up. As he makes his way through the vortex, he spots billions and billions of monsters headed towards them.

Sage examines the black box some more and discovers the true origins of the monsters. The Celestials created life because they were lonely but then created destruction to balance things out. However, the monsters turned on the Celestials and, having no way to kill them, they copied the universe again and again and then imprisoned the monsters in-between them. Now, billions of years later, the heroes' constant travelling between realities has weakened the dimensional walls and allowed the monsters to escape. Nightcrawler appears and Sage tells him she knows what to do.

In New Apocalypse, Jean breaks off her battle against the monsters and rejoins to group. She warns them that sooner or later she will fall and then the whole universe is done for. Just as concerning to the group, however, is that it is apparent that, though she is holding on, the Apocalypse power is taking her over. Just as she departs to continue her battle, Nightcrawler and Sage appear and explain what they have learned. Prophet and Sage explain that the only way to beat the monsters is to imprison them again, but it is too late to find an empty universe, so that the prison must be the one they are in. As more monsters begin pouring through the portal, they try to figure out a way to get everyone to the 616 world and to close the portal off. Nightcrawler tells them he can do both.

Dazzler disagrees with the plan, as she doesn't want to sacrifice the world, but Prophet convinces her it's the only way. Prophet has Nightcrawler teleport people to the 616 universe, starting with Harper Simmons. On the other side, Nightcrawler pauses just long enough to apologize to Wolverine for betraying X-Force before taking the next trip.

After transporting Sage, Nightcrawler goes to transport Iceman. However, just as he is about to teleport, Dark Beast breaks free of his restraints and teleports with them. Inside the portal, he secretly jams a small piece of the Apocalypse seed in Iceman's ice for safe keeping. When they materialize on the other side, Gambit knocks Dark Beast out as the others tend to Iceman. As Nightcrawler next fetches Howlett and Cyclops, Dazzler and Prophet are the last ones. As they watch Jean fight the monsters, Dazzler tries to convince him to go back with them but he refuses, explaining that he has been fighting for the world most of his life and he won't leave it.

The decision is taken away from him, however, when Jean blasts him from behind and knocks him out. She tells her to get Prophet to the 616 universe as the world is a better place with him in it. Nightcrawler appears and takes Dazzler and Prophet through the portal, leaving Jean to continue battling the increasing hoard of monsters.

On the other side, as everyone tries to figure out how to close the portal, Nightcrawler realizes that he can grab hold of the fragments of the barrier within the portal and teleport them back to the Apocalypse universe, thereby closing the portal. Sage tells him he will be killed but he doesn't care and teleports up to the portal. As he grabs the barrier pieces, they start to drain his life force. With the last bit of his power, he teleports through the portal, causing it to close behind him.

In the aftermath, Dark Beast is taken away by the authorities whilst Howlett and Cyclops decide to head to Greece to get Hercules out of the underworld. Wolverine offers Dazzler a job at the school but she turns him down, saying she needs some time alone. A few weeks later, she sits on a beach and creates a giant holographic image of her fallen friends. On another beach, Harper Simmons and Prophet have a drink to commemorate their own fallen teammates.