Plot

Jean Grey begins to develop incredible powers that corrupt and turn her into a Dark Phoenix, causing the X-Men to have to decide if her life is worth more than all of humanity.

The X-Men. Protectors of peace. Jean Grey is one of the most beloved X-Men. But when a mission goes wrong, Jean is exposed to a dark and ancient power. This power has destroyed everything it comes in contact with, until her. Now that this power is becoming unstable, she releases it with destruction and anger. Now that this foreign power is consuming her, and the world is threatened, the X-Men have to face an important truth: they must save either the world, or their friend who threatens it.

After a car accident with her parents caused by her powers, the girl Jean Grey is raised by Professor Charles Xavier at his school. Years later, the X-Men are assigned to rescue a team of astronauts and Jean Grey is exposed to a dark and evil force. Soon her behavior changes affecting her friends and the future of the world.—Claudio Carvalho, Rio de Janeiro, Brazil

Almost one long and peaceful decade after the nearly disastrous events in X-Men: Apocalypse (2016), Professor Charles Xavier's X-Men enjoy a mutually beneficial pact with the humans, undertaking increasingly dangerous missions as the Earth's valiant protectors. However, during a risky NASA crisis in 1992, Jean Grey is imbued with an unfathomable cosmic force--and even though she should be dead--this inexhaustible but uncontrollable energy enhances her already formidable abilities, transforming her into an all-powerful being. More and more, Jean's otherworldly powers become unpredictable and destructive, posing a direct threat not only to herself and her companions but also to the entire world. Is humankind prepared for the rise of the Dark Phoenix?—Nick Riganas

In 1975, eight-year-old Jean Grey (Sophie Turner) inadvertently uses her Telekinesis to cause a car accident that kills her mother (Hannah Emily Anderson), and apparently, her father John Grey (Scott Shepherd). During the crash, Jean formed a protective energy barrier around herself, which protected her from any harm. Shortly afterwards, Professor Charles Xavier (James McAvoy) takes her to Xavier's School for Gifted Youngsters, where he mentally blocks the accident from her memories and helps her hone her psychic abilities.

In 1992, nine years after the worldwide devastation caused by En Sabah Nur, the X-Men respond to a distress signal from the space shuttle Endeavour, which is critically damaged by a solar flare-like energy. Due to this, the Space shuttle has lost all control and is not even in a position to return to Earth. While the X-Men (Jean, Hank, Peter, Kurt Wagner / Nightcrawler (Kodi Smit-McPhee), Scott Summers / Cyclops (Tye Sheridan) and Raven) save all of the astronauts, one person is left behind in the airlock of the shuttle with a mysterious alien energy cloud moving towards it. Raven wants to return to Earth, but Xavier asks for everyone to be extracted. Kurt and Jean go back in to retrieve the stranded astronaut. Kurt escapes with the astronaut, but Jean is hit by the energy cloud. As the cloud moves towards the X-Plane, Jean absorbs the entire energy cloud into her body to save the X-Men's aircraft from destruction. Jean survives the event, and her psychic powers are greatly amplified as a result.

Raven is livid at Xavier for putting the team in greater and greater dangerous situations. She believes that Xavier is now used to the spotlight and is willing to sacrifice the mutants for his own glory. Xavier says that using mutants to save humans is the only way to make sure that humans never see mutants as a threat. Raven tells Hank that they are living Xavier's life and not their own. Raven and Hank are the only mutants left from the First Class. At the same time the mental block placed by Xavier is destroyed and she accidentally attacks the mutants celebrating at Xavier's school after a mental breakdown. She later travels to her childhood hometown of Red Hook, New York after discovering that her father is still alive.

Jean meets John and realizes that John knew that she survived the crash, but he refused to take her back as John was afraid of mutants. Jean believes that Charles deceived her by not telling her the truth and used her for her powers. The X-Men attempt to take Jean back to the school, but she injures Peter Maximoff / Quicksilver (Magneto's son) (Evan Peters) and accidentally kills Raven Darkholme/Mystique (Jennifer Lawrence) and several local police officers before flying away. Hank is very angry with Charles's manipulating nature and is even more angry when Charles refuses to accept his mistakes.

Jean travels to the mutant refugee island of Genosha to seek assistance from Erik Lehnsherr / Magneto (Michael Fassbender) in controlling her powers but is turned away by Erik after she engages in combat with U.S. military forces tasked with her arrest.

Jean meets Vuk (Jessica Chastain), the leader of a shape-shifting alien race known as the D'Bari (Vuk had landed on Earth a few days ago after Jean had absorbed the cosmic energy and took the shape of the first human she encountered) in New York city. Her shape Shifters had briefed Vuk that Jean was troubled due to the powers within her and was anguished that due to the uncontrollable nature of the power, she is killing the people she loves. Vuk explains to Jean that she has been possessed by a force of cosmic power which wiped out the D'Bari home planet years ago. The power had consumed all those it came across until it was drawn to Jean. By controlling this power, Jean can shape planets to bear life and become the most powerful being in the galaxy. Meanwhile, Hank McCoy / Beast (Nicholas Hoult), who feels betrayed by Xavier's manipulation of Jean's memories, allies with Erik and the mutant refugees in an attempt to put Jean down in New York City. Erik is angry as Jean never told her that she killed Raven, so he is keen on revenge.

Upon learning of Erik's plan to kill Jean, the X-Men confront him and his faction in New York City. As they battle, Erik manages to infiltrate the building and confront Jean, but he is overpowered by her amplified abilities, and is knocked out by her attack. Xavier then enters the building with Night Crawler (Kodi Smit-McPhee). Jean attacks them until Xavier convinces her to read his memories - allowing her personality to resurface. Feeling remorseful, Jean asks Vuk to take the force from her; however, it quickly turns out that doing so would kill her. Xavier and Scott / Cyclops (Tye Sheridan) are able to prevent Vuk from fully absorbing the force from Jean, though Jean loses consciousness from the ordeal. However, just then, the mutants are captured by troops sent by the U.S. government, who utilize stun weapons and power-disabling collars, and then the mutants from both factions are placed on a train headed towards a mutant containment facility.

A remorseful Xavier admits to a very resentful Hank that the latter was right in his earlier accusations of violating Jean's mind and lying to her. The train is attacked by Vuk and her D'Bari forces. When the soldiers are overpowered by the shape Shifters, the mutants are freed from their restraints to combat the threat. After the mutants take out most of the D'Bari attackers Vuk arrives and heads for Jean, defeating each mutant that attempts to stop her. Meanwhile Xavier confers with Jean within his mind, allowing Jean's personality to gain control of the force within her. After forgiving Xavier, Jean saves the mutants from Vuk's attacks and the ensuing train wreck before proceeding to easily vaporize the remaining D'Bari aliens when they attack her. Vuk once again attempts to drain Jean of the force, but Jean takes Vuk into outer space and retakes the power that the D'Bari had stolen earlier, then kills Vuk. Jean then disappears as her full potential is unleashed releasing a burst of energy in the form of a phoenix.

In the aftermath of the incident the Xavier's school is renamed the "Jean Grey School for Gifted Youngsters", and Hank becomes their new dean with Xavier having retired after decades of fighting for mutant rights. While settling himself in Paris, Xavier is reunited with Erik and reluctantly agrees to play a game of chess with him. As they start playing, a flaming phoenix appears in the sky.