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Riri Williams is a Chicago genius who was studying at MIT. She gets expelled for selling her work to other students and comes home with a suit of armor she built herself. Riri still has not gotten over the deaths of her best friend, Natalie, and her stepfather, Gary, both killed in a drive-by shooting. Back in the neighborhood, she reconnects with Natalie's brother, Xavier, and meets Parker Robbins, leader of a crew of thieves calling themselves The Hood, who wears a mysterious cloak with powers.
Short on cash to upgrade her suit, Riri agrees to take part in three of the crew's heists. Parker sells the gang as a kind of Robin Hood outfit, hitting companies that hurt the community. To help with the work, Riri maps her own brain and builds an AI for the suit. Because of her grief, the AI comes out with Natalie's face and personality. During one job, Riri blackmails an arms dealer named Joe McGillicuddy, who is really Zeke Stane, Obadiah Stane's son.
Things fall apart when Riri cuts a piece off Parker's cloak. The alarm goes off, Parker kills an executive, and a crew member suffocates. Parker blames Riri, and she becomes the traitor in everyone's eyes. Her mother, Ronnie, takes the scrap of cloak to Zelma, a young sorcerer who works at a magic shop, and learns the fabric comes from a dark dimension and is messing with Parker's mind. At the same time, Xavier finds the Natalie AI and demands Riri delete it.
Parker sends the crew to kill Riri. The gang breaks Zeke out of prison, and Parker uses the cloak to take control of his body. Riri makes up with her mother, Xavier, and the AI, and together they build a new suit in Gary's garage, with Zelma's magic powering it. The Natalie AI is lost in the process. In the final fight, Riri rips the cloak off Parker and he is left powerless.
Then the one behind the cloak all along shows up: Mephisto. He offers Riri a deal, the chance to get back someone she lost. Riri accepts. In the last scene, she hugs Natalie, alive again, while a mark of the demon creeps up her arm. In the post-credits scene, Parker goes to the magic shop to ask Zelma for help getting some even bigger magic.