In Brightburn, Kansas, married farmers Tori and Kyle Breyer struggle with painful infertility. One night, an alien pod crashes through the trees on their property. Inside the glowing vessel, they discover an unharmed infant boy. Believing their prayers have been answered, Tori and Kyle hide the spaceship beneath the wooden floorboards of their barn and adopt the child, naming him Brandon. Twelve years later, Brandon is an intelligent and obedient boy who has never suffered an illness, cut, or broken bone. Around his twelfth birthday, the hidden alien craft begins glowing with red light and emitting high-frequency alien transmissions directly into Brandon's mind. Brandon starts sleepwalking toward the barn, drawn by an irresistible command to "Take the world." Brandon soon discovers his godlike physical abilities. When his father tells him to start the lawnmower, Brandon throws the machine across the yard and catches the spinning steel blade with his bare hand without a scratch. When a classmate named Caitlyn refuses to help him up during physical education, calling him a creep, Brandon crushes her hand during trust falls, snapping her bones. Caitlyn's mother Erica, who works at a local diner, demands that the school punish Brandon and bans her daughter from speaking to him. That night, Brandon puts on a handmade red cloth mask and cape and stalks Erica at the diner. Brandon shatters the windows, causes a shard of glass to slice deep into Erica's eyeball, and brutally executes her with superspeed and laser heat vision, tearing the building apart. Brandon's uncle Noah scolds Brandon for inappropriate behavior toward Caitlyn and threatens to report him. Later, as Noah drives his pickup truck home, Brandon appears in the sky. Brandon lifts the entire truck into the air and drops it onto the highway. Noah's face is slammed violently into the steering wheel, shattering his jaw into exposed bone and teeth before he dies in agony. Kyle begins suspecting that Brandon is behind the murders after finding Brandon's bloodstained symbol drawn in the woods. To protect his family, Kyle takes Brandon on a hunting trip into the forest and attempts to shoot the boy in the back of the head with a rifle. The high-powered bullet shatters against Brandon's skull. Brandon turns around and obliterates Kyle's head with dual red heat-vision beams. Brandon returns home and goes on a rampage, tearing the house apart while police officers try in vain to shoot him. Brandon snaps the deputy's neck and melts the sheriff's face with laser eyes. Tori flees into the barn, where she discovers Brandon's sketchbook filled with gruesome drawings of his kills and his alien crest. Realizing that the jagged metal of the alien spaceship is the only material on Earth sharp enough to cut Brandon's skin, Tori breaks off a sharp metal shard and hides it behind her back. When Brandon enters the barn, Tori tearfully embraces him, telling him she loves him and will always protect him. But as she raises the alien shard to stab his heart, Brandon's superhuman reflexes catch her arm. Brandon looks into his mother's eyes with cold, emotionless detachment. Brandon grabs Tori, rockets her thousands of feet into the sky above the clouds, and drops her. Tori plummets to her death, her body shattering upon impact on the farm below. An airliner passing overhead crashes into the farmhouse, killing all passengers and destroying the evidence of the family's murder.

In the aftermath of the disaster, young Brandon sits wrapped in a shock blanket, calmly eating cereal while emergency crews sift through the smoking rubble of the airplane crash. Authorities believe the crash was a tragic accident that wiped out Brandon's parents, leaving Brandon as the sole heroic survivor. During the mid-credits sequence, news reports reveal that a terrifying masked creature has been destroying infrastructure across the country, leveling a corporate high-rise and slaughtering dozens of innocent people. Michael Rooker cameos as a crazed internet conspiracy vlogger named "The Big T", broadcasting from a secret basement bunker. He warns the public about Brandon's reign of terror and reveals that other super-powered monsters are emerging around the globe, including a half-human sea creature who sinks ships and a witch who chokes men with a glowing rope. The movie closes on a chilling shot of Brandon hovering in the night sky over a burning metropolis, wearing his red hood as sirens scream below. The ending makes it crystal clear: there is no hero coming to save the world. Brandon is not conflicted or misunderstood. He is an apex alien predator fulfilling his genetic directive to conquer and destroy humanity.