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NOTE: This takes place after Avengers: Wrack of Dormammu.
The human member of the Nova Corps known as Sam Alexander
In the MCU dimension, the human member of the Nova Corps known as Sam Alexander flies to the Terrax´s planet to warn him about the Phoenix Force, an entity from space that wants to take control over the whole universe. When he faces Terrax, the Phoenix Force appears, kills Terrax and creates chaos on Terrax´s planet. The Phoenix Force reveals its plans to take control over this universe by taking control of her avatar on Earth-108855 (which is Jean Grey). After Nova scaped, the Phoenix Force destroys Terrax´s planet, and the title of the movie appears.
Years after the events of X-Men: Apocalypse (from 2016 to 2026) , the Earth-108855 (the Earth where all the X-Men Movies after X-Men: Apocalypse, take place) is merged into the apocalypse by the fault of Dark Phoenix. A genetical descendant of Scott Summers/Cyclops, Nathan Summers/Cable, alongside the mutants Colossus, Deadpool, Iceman, Sunspot and Negasonic Teenage Warhead are fighting a group of undead mutants brought back to life by Dark Phoenix. After defeating an undead Wolverine, Deadpool asks why the world is ruled by Dark Phoenix. Cable says that a group of invaders that came from another world named "Avengers" allied with Dark Phoenix and killed the X-Men, and also helped on the mutant genocide. Suddenly, Dark Phoenix appears and sends a dark fire blast to the group, and Colossus orders his friends to go away, and then, Colossus receives the dark fire blast, and he falls dead to the ground, and Dark Phoenix held his body into the air and disintegrated him.
After that, the group is led by Cable to their secret shelter. Cable actives a time machine and travels to the past alongside Negasonic and Deadpool. They arrived to 1985, two years after the X-Men formed in 1983 during "X-Men: Apocalypse". They travel to Xavier Academy, where they find Profesor Charles Xavier, Cyclops, Jean Grey, Beast, Mystique, Quicksilver, Storm, and Nightcrawler, and they warn them about the Avengers and Dark Phoenix, so the X-Men decided to return to the action. Also, Kitty Pride, a mutant with the power of telekynesis and teleportation, asks Cable to join to the team, and he accepts. Wanda Maximoff, Quicksilver´s sister, decides to join to the team. Since Cable learned that Wanda´s Chaos Magic powers are unestable, he designed a device to control her if she gets out of control. The X-Men are conducted to an interdimensional portal built by Cable, and he actives it. The X-Men are teleported one by one to the MCU Universe.
Meanwhile, in the MCU Universe, the Avengers decided to divide themselves after the events of Wrack of Dormammu.
- Robert Downey Jr. as Tony Stark/Iron Man
- Scarlett Johansson as Natasha Romanoff/Black Widow.
- Mark Ruffalo as Bruce Banner/Hulk.
- Chris Hemsworth as Thor.
- Paul Rudd as Scott Lang/Ant-Man.
- Tom Holland as Peter Parker/Spider-Man.
- Brie Larson as Carol Danvers/Captain Marvel.
- Paul Bettany as Vision.
- Chadwick Boseman as T´Challa/Black Panther.
- Don Cheadle as James Rhodes/War Machine.
- Evangeline Lily as Hope van Dyne/Wasp.
- Ezra Bridger as Francis Barton/Hawkeye.
- Benedict Cumberbatch as Stephen Strange/Doctor Strange.
- Chris Evans as Steve Rogers/Captain America.
- Dylan O´Bryan as Sam Alexander/Nova.
- Tye Shediran as Scott Summers/Cyclops.
- James McAvoy as Profesor Charles Xavier.
- Sophie Turner as Jean Grey/Phoenix.
- Nicholas Hoult as Hank McCoy/Beast.
- Jennifer Lawrence as Raven Darkholme/Mystique.
- Evan Peters as Peter Maximoff/Quicksilver.
- Kodi Smit-McPhee as Kurt Wagner/Nightcrawler.
- Alexandra Shipp as Ororo Munroe/Storm
- Michael Fassbender as Max Eisenhardt/Erik Leighenn/Magneto.
- Oliva Munn as Psylocke.
- Ellen Page as Kitty Pryde/Shadowcat.
- Shawn Ashmore as Bobby Drake/Ice-Man.
- Adan Canto as Roberto da Costa/Sunspot.
- Elizabeth Olsen as Wanda Maximoff/Scarlet Witch.
- James Spader as Onslaught. (Voice).
- Oscar Isaac as Dormammu. (Voice).
- Famke Janssen as Phoenix Force and Dark Phoenix. (Voice).
- Robin Atkin Downes as Terrax.
- Kurt Russel as Cable.
- Stefan Kapicic as Peter Rasputin/Colossus.
- Ryan Reynolds as Wade Wilson/Deadpool.
- Brianna Hilderbrand as Ellie Hiciste/Negasonic Teenage Warhead.
- Eleanor Tomlinson as Laura Kinney/X-23.
- Hugh Jackman as Undead Wolverine. (Voice). (Unacredited).