In 2006, a spaceship crashes in a forest in Brightburn, Kansas. Tori and Kyle Breyer, a couple who are trying but unable to conceive, see an explosion from their house and go out to the crash site. When they reach the crash they find a spaceship with a baby inside, whom they adopt and name Brandon. The spacecraft is then hidden in their barn, which Brandon's parents tell him to stay away from. Around Brandon's 12th birthday, he begins to discover he has exceptional strength, as well as the ability to fly. It is revealed that Brandon has never bled or received a bruise. His disposition also changes, from a sweet child to having anger and temper issues.
In middle school, Brandon is an academically exceptional student but socially awkward and frequently bullied. Caitlyn Connor, a girl in Brandon's class, shows sympathy for him and tells him that smart people end up ruling the world. Brandon develops a crush and stalks Caitlyn in her room in the middle of the night, scaring her. One day, during a trust exercise in the Physical Education class, Brandon falls towards Caitlyn, but she lets him drop to the ground and calls him a pervert. Angered by her accusation, Brandon crushes her hand and is suspended from school after being accosted by Caitlyn's mother Erika.
That night, Brandon sleepwalks once again to the spaceship and breaks into the barn where it is hidden, cutting his hand on the ship. Tori follows and sees him levitating, chanting the ship's message: "Take the world". Tori reveals the truth of his origin, and Brandon angrily trashes the house and later kills Erika. Police find a series of symbols drawn on a window while investigating Erika's disappearance, the same symbols that Brandon is seen drawing in his notebook. The next day, Brandon dons his costume and is then caught sneaking into his aunt and uncle's house by his Uncle Noah. After Noah threatens to tell his father, Brandon kills him by lifting and dropping his vehicle. The following morning, Tori and Kyle inform Brandon of Noah's death, which the police believe was an accident, but Brandon shows no emotion or empathy. Suspecting Brandon to be the killer, Kyle finds Brandon's blood-stained shirt and shows it to Tori, but she refuses to believe it.
Kyle takes Brandon on a father-son hunting trip in the woods. Kyle attempts to shoot and kill Brandon with his hunting rifle, but the execution fails when the bullet bounces off the back of Brandon's head. Brandon then kills him with heat vision. A sheriff arrives at the Breyers' and asks to see Brandon. Tori tells the sheriff that Kyle and Brandon are not home, so he shows Tori the symbol found at the scenes of Erika's and Noah's deaths. Tori finds Brandon's notebook with drawings of his murders, the same symbol that the sheriff found at both murders and his message to "take the world". She begins to believe Kyle. She tries calling Kyle, but Brandon answers and implies he is now coming for her.
Brandon returns and begins destroying the house. Tori calls 911, but both the sheriff and his deputy are killed by Brandon before backup can arrive. Remembering that the ship's hull can cut Brandon, Tori runs to the barn and discovers Erika's eviscerated body. As Brandon pursues her, Tori tries to tell him about how much she loves him. She then tries to stab him with a piece of the ship, but Brandon grabs her arm, tosses the shard to the floor, and flies his mother into the sky to drop her from several thousand feet to her death. After he watches her fall, he looks up to see an oncoming airplane.
The following morning, the airplane is revealed to have mysteriously crashed into the farmhouse, destroying the evidence of the murders from the previous night. Brandon's symbol is seen painted on a piece of the plane wreckage.
In a mid-credits scene, Brandon, now identified as Brightburn, is seen from the perspective of news reports and eyewitness footage over the locations of various disasters, such as a building collapse and a wildfire. An online conspiracy theorist posts his own video analyzing the disasters and referencing attacks by other cryptids elsewhere in the world.
