Teenage science genius Paul Conway (Matthew Laborteaux) and his single mother Jeannie (Anne Twomey) move into their new house in the town of Welling. He soon becomes friends with newspaper delivery boy Tom Toomey (Michael Sharrett). Living next door to Paul is Samantha Pringle (Kristy Swanson) and her abusive, alcoholic father Harry (Richard Marcus). Paul built a robot named BB (Charles Fleischer), which occasionally displays autonomous behavior, such as being protective of Paul. Paul, Jeannie, and BB meet Paul's professor, Dr. Johanson (Russ Marin), at Polytech, a prestigious university where Paul has a scholarship.
One day, Tom, Paul and BB stop at the house of reclusive harridan Elvira Parker (Anne Ramsey), who threatens the boys with a shotgun and expresses instant dislike for BB. Walking away, the trio then encounter a motorcycle gang led by bully Carl (Andrew Roperto). When Carl intimidates Paul, BB assaults him. Another day, while playing basketball, BB accidentally tosses the ball onto Elvira's porch. She takes the ball away from them refuses to give it back, with BB's glowing red over her hostile attitude. On Halloween night, Tom decides to pull a prank on Elvira with the help of Paul, Samantha and BB. BB unlocks her gate and Samantha rings her doorbell. When alarms go off, they hide in a shrubbery nearby. When Elvira sees BB standing near her porch, she shoots him with her shotgun, destroying him and devastating Paul.
On Thanksgiving, Paul and Samantha share their first kiss. Samantha returns home late at night, outraging her father, who slaps her and pushes her down the stairs. At the hospital, Paul learns that Samantha is brain-dead and will be on life support for 24 hours before the plug is pulled. As BB's microchip can interface with the human brain, Paul decides to use it to revive Samantha with Tom's help. The boys enter the hospital using a key taken from Tom's father, who works there as a security guard. After Tom deactivates the power from the basement, Paul takes Samantha to his lab. He inserts the microchip into Samantha's brain and takes her back to his house, hiding her in the shed. After he activates the microchip, Samantha "wakes up", but her mannerisms are completely mechanical, suggesting BB is in control of her body.
In the middle of the night, Paul finds Samantha staring at the window, looking at her father, and he deactivates her. The next morning, Paul finds Samantha gone. When Harry finds the cellar door open and goes downstairs, Samantha attacks him, breaks his wrist and snaps his neck. Paul finds Samantha, and Harry's corpse, in the cellar. Horrified, he hides the body, takes Samantha back to his home and locks her in his bedroom. At night, Samantha breaks into Elvira's house and corners her by throwing her to the wall of her living room. As Elvira screams in horror, Samantha kills her by exploding her head with the basketball she stole from Tom.
When Tom learns of Samantha's rampage, he gets into a fight with Paul and threatens to call the police. Still being protective of Paul, Samantha jumps out the attic window and attacks Tom, with Paul and Jeannie intervening. Trying to get her under control, Paul slaps Samantha, resulting in her strangling him. Samantha, quickly coming to her senses, then lets him go and runs away. As Paul goes after her, he again encounters Carl, who gets into a fight with him. Samantha goes back for Paul, grabs Carl and kills him by throwing him at an incoming police car. She runs back to Paul's shed, where Paul comforts her and realizes she's regaining some of her humanity. However, the police arrive with their guns aimed at Samantha, who yells out Paul's name in her human voice. She runs towards him, trying to protect him, but Sergeant Volchek (Lee Paul), thinking she's trying to attack him, shoots her. She says Paul's name one more time before dying in his arms.
Later at the morgue, Paul tries to steal Samantha's body once more. Suddenly, Samantha grabs Paul's neck and her face rips apart, revealing a terrifying variant of BB's head. Her skin strips away, revealing half-robotic bones underneath. With a robotic voice, Samantha tells him to come with her. When a horrified Paul screams and refuses to do so, she snaps his neck, killing him.
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