Plot

Five young mutants, just discovering their abilities while held in a secret facility against their will, fight to escape their past sins and save themselves.

After a near-death experience during a strange tornado, Danielle Moonstar awakens in an abandoned research facility run by the mysterious Dr Cecilia Reyes. There, Dr Reyes introduces Dani to four other equally uncommon teenagers: Illyana Rasputin, Rahne Sinclair, Sam Guthrie, and Roberto da Costa, who wants to keep them safe and sound until they learn how to be in full control of their extraordinary abilities. However, even though the exceptional team of traumatised inmates believes that they're being treated and cared for, before long, they all start to experience horrifying hallucinations. But, this institution was supposed to be a safe place. Are they patients or prisoners?—Nick Riganas

Danielle "Dani" Moonstar / Mirage (Blu Hunt), a young Cheyenne Native American, is hidden in a tree by her father as her entire reservation is devastated by an apparent tornado (even though Dani remembers hearing growling noises throughout the ordeal), leaving her the only survivor. After falling unconscious, Dani awakens in an oddly empty hospital run by Dr. Cecilia Reyes (Alice Braga) (A strict mentor to the group and a medical doctor who can generate protective fields around herself), who comforts Dani, explaining that she is not an ordinary human being, but rather has unique mutant DNA (who can create illusions based on the fears and desires of other people), and advises her to remain in this hospital until she learns the effects and how to control them.

Dani is introduced to four other teenagers; Samuel "Sam" Guthrie / Cannonball (Charlie Heaton) (can propel himself into the air, and is invulnerable while doing so), Illyana Rasputin / Magik (Anya Taylor-Joy) (A Russian mutant with sorcery powers, she can manifest the Soulsword and use Teleportation discs to travel), Roberto "Bobby" Da Costa / Sunspot (Henry Zaga) (A Brazilian mutant, who comes from one of the richest families and who can manipulate solar energy) and Rahne Sinclair / Wolfsbane (Maisie Williams) (A Scottish mutant who can turn into a wolf and is struggling to reconcile this with her religious beliefs)

Reyes has brought each of them to the hospital, after they have each either experienced, or accidentally caused, a horrible tragedy; Sam collapsed an entire coal mine on his father and coworkers killing them all, Roberto accidentally burned his girlfriend to death (he gets too hot during sex), Rahne escaped her devoutly Catholic village by killing the priest after he branded her as a witch, and Illyana was enslaved and sexually abused as a child. Reyes herself is also a powerful mutant who can manipulate "plasma-energy" force fields, preventing the five of them from leaving the facility. Reyes tells the teenagers that the nearest town is 20 miles away, but the teenagers know that she could be lying.

Collectively, the five teenagers believe they are being trained to join the X-Men, hence the strict supervision. Reyes warns them that they are considered dangerous and should not leave until they have mastered their superhuman abilities. Dani attempts to escape but is stopped by a force field surrounding the entire hospital grounds. Illyana is mean towards the new girl Dani, but Rahne is sympathetic to her and makes friends with Dani. Dani then plans suicide from the church clock-tower but is prevented by Rahne. The two begin to form a romantic relationship, but on the other hand Illyana antagonizes Dani, who discovers that Illyana has a hand puppet of a purple dragon called Lockheed. Illyana even attacks Dani with her Soulsword when Dani tries to stand up to her taunts and insults. Soon, the group all begin to have horrifying real visions of their past tragedies, which results in Rahne getting branded on the neck by the same Priest whom she had previously killed, Sam revisits his dad in the mines and is attacked by all the miners, Dani herself wakes up one night covered in blood after she has a dream about the night her tribe was attacked.

In one group session in the attic, Dani reveals that as a kid she was afraid of the dark. So, her father told her the legend of the demon bear, which resides in each one of us. As a child, the bear is small and weak, but as we grow, our fears grow as well, and this feeds the bear. Dani's father told her never to be afraid. One night, Rahne shows Dani that she has found a way outside the hospital building, through the ventilation vents. Even the hospital building doors are access controlled and the teenagers need permission to venture outside. Dani and Rahne lay in the gardens and share an intimate moment. Illyana and Roberto go for a swim together in the swimming pool, as they start to have sex, Illyana disappears and is replaced by the burned version of Roberto's girlfriend. Roberto is alight with fire, which Reyes brings under control by pushing him into the pool. Roberto accuses Illyana of trying to kill him, but Reyes insists that Illyana was in her room.

The mutants are under constant AI supervision and Reyes determines that the nightmares are happening at night, when Dani is asleep and her Psionic energy levels are highly elevated. Both Illyana and Reyes deduce that the visions are the result of Dani's powers manifesting themselves; the ability to physically manifest fears, illusions, or other thoughts from a person's mind. Reyes consults her employers, the Essex Corporation, who instruct her to collect Dani's DNA and then euthanize her.

As Reyes straps her down on a gurney, Dani's panic causes her powers to go haywire. Illyana and Sam are attacked by physical manifestations of Illyana's childhood abusers - monstrous humanoid creatures called the "Smiling Men" - while Roberto tries to break through the hospital's outer barrier, which has now contracted in diameter. Dani uses her powers to discover Reyes's true intentions before Rahne arrives in half-wolf form and mauls Reyes, forcing her to flee.

The five regroup in Reyes's office and realize that Reyes was training them to be assassins for Essex, and in order to escape, they must kill Reyes to deprive the force fields of their power source. They find and confront Reyes, who warns them that Dani is too powerful and will destroy them all. Reyes restricts them all with force fields and tries to again kill Dani by asphyxiating her with a force field, which unleashes the Demon Bear - Dani's own fears manifested through her power, and the true cause of her reservation's destruction - on her; Reyes is devoured, and Dani rendered unconscious.

Rahne tries to reach through to Dani's subconscious and urges her to wake up, while Illyana uses her powers to travel to "limbo" where she retrieves a glowing sword, armor and a tiny, physical manifestation of Lockheed to battle Demon Bear. Eventually, Sam and Roberto join the fight, as well as Rahne, all to no avail. Dani is visited by her father's spirit, who encourages her to face her fear; she awakens and confronts Demon Bear, calming and thus dissipating it. As day breaks, the group leaves the now without shield facility to find the nearest town.