In the future, robots known as Sentinels are exterminating mutants, and humans that dared to assist the mutants and harbor genes that lead to mutant offspring. A band of mutants manages to evade the Sentinels with the assistance of Kitty Pryde, who can project a person’s consciousness back in time to deliver warnings. Pryde’s group convenes with Storm, Wolverine, Professor Xavier, and Magneto in China. Pryde sends Wolverine’s consciousness back to 1973 to prevent Mystique from assassinating Bolivar Trask, the creator of the Sentinels. Following the assassination, Mystique was captured and her DNA used by Trask’s company to create Sentinels that are near-invincible due to their ability to adapt to any mutant power. Xavier and Magneto advise Wolverine to seek out both of their younger selves for aid.

Wolverine wakes up in 1973 and travels to the X-Mansion, where he encounters Beast and Xavier. Beast reveals that Xavier’s Institute for Gifted Youngsters closed after most of the teachers and students were drafted for the Vietnam War. A depressed Xavier has been abusing a serum which Beast created to enable him to walk, causing his telepathic powers to be suppressed as well. Wolverine explains his mission and persuades Xavier to help free Magneto from a prison cell beneath the Pentagon, where he is being held for allegedly killing President John F. Kennedy. They succeed with the help of Peter Maximoff, a mutant with superhuman speed.

In Washington D.C., Trask unsuccessfully lobbies for his Sentinel program with Congress. Meanwhile, in Saigon, Mystique prevents William Stryker from appropriating a group of mutant G.I.s for Trask’s research. Mystique investigates Trask’s office and discovers he has been capturing mutants for experiments. Xavier, Magneto, Beast, and Wolverine fly to Paris to intercept Mystique. In Paris, Mystique impersonates a Vietnamese general to infiltrate a meeting with Trask, who is attempting to sell his Sentinel technology to communist nations. Xavier, Magneto, Beast, and Wolverine arrive as Mystique is about to kill Trask. To ensure that Mystique’s DNA can never be used for the Sentinels, Magneto tries to kill her, but she flees out of a window. The fight spills onto the streets and in front of the public, allowing both Magneto and Mystique to escape.

Although Trask is saved, the world is horrified by the existence of mutants. President Richard Nixon approves Trask’s Sentinel program and arranges an unveiling. Trask’s scientists recover Mystique’s blood from the street. Meanwhile, Magneto—who has recovered his telepathy-blocking helmet—intercepts the prototype Sentinels in transit and laces their polymer-based frames with steel, allowing him to control them. At the mansion, Xavier stops taking his serum and slowly regains his mental powers while losing the ability to walk. Through Wolverine, Xavier speaks to his future self and is inspired to struggle for human-mutant peace once again. He uses Cerebro to track Mystique, who is heading to Washington.

Nixon unveils the Sentinel prototypes at the White House as Xavier, Wolverine, and Beast search for Mystique. Magneto commandeers the Sentinels and attacks the crowd, then sets a stadium around the White House as a barricade. Nixon and Trask are taken to a safe room, accompanied by the Cabinet, Secret Service officers, and Mystique disguised as a Secret Service member. Wolverine and Beast try to stop Magneto, but he pits a Sentinel against them and throws Wolverine into the Potomac River. In the future, the X-Men make their final stand as a large army of Sentinels attacks the monastery. In 1973, Magneto pulls the safe room out of the White House and prepares to kill Nixon and his Cabinet in front of the world. Mystique, disguised as Nixon, shoots Magneto with a plastic gun. Xavier persuades Mystique not to kill Trask and allows her and Magneto to flee. Mystique’s actions are seen as a mutant saving the President, leading to the cancellation of the Sentinel program. Trask is arrested for attempting to sell American military secrets.

Wolverine wakes up in the future and finds Iceman, Rogue, Colossus, Pryde, Beast, Storm, Jean Grey, Cyclops, and Xavier alive. Back in 1973, Mystique, impersonating Stryker, takes custody of Wolverine after he is pulled from the river.

In a post-credits scene, a multitude of people gathered in a desert pray to En Sabah Nur, who is using telekinesis to construct a pyramid as four horsemen keep watch nearby.