Outside a restaurant named 'Burger Heaven', a thief attempts to break into a Volkswagen van, but his efforts are thwarted by a robot named BB, voiced by Charles Fleischer. After briefly incapacitating the thief, BB lets him go. Soon after, Paul Conway, played by Matthew Labyorteaux, and his mother Jeannie (Anne Twomey) are seen returning from shopping and driving away in the same van. Paul is the creator of BB, a robot he built himself. The next day, they arrive at their new home in the quaint town of Welling, Pennsylvania.
As they drive to their new residence, Jeannie confirms the address, and once they’ve unpacked, a neighbor boy named Tom Toomey (Michael Sharrett), a local paperboy, falls from his bike. Curious about BB, he approaches Paul, who explains that he programmed the robot. Paul possesses an exceptional intellect, landing a university scholarship due to his strong passions for neurology and artificial intelligence. However, after a long day, BB's battery runs low, and the robot autonomously seeks a power source to recharge. At the university, the trio meets Dr. Johanson, Paul's mentor, who offers them a comprehensive tour of the laboratory.
Days later, while Paul and BB are tidying up the yard, they meet Samantha Pringle, portrayed by Kristy Swanson, who lives next door. Paul can’t help but notice bruises on her arms, which Samantha tries to conceal. Trouble arises as her abusive father, Harry (Richard Marcus), comes out, casting a threatening shadow over the frightened Samantha. Later, she is drawn to Paul and Jeannie’s household for solace, but her father soon arrives to drag her back home and brutally beats her.
With time, Tom joins Paul in teaching BB how to deliver newspapers. One day, they unwittingly venture to the home of the reclusive Elvira Parker (Anne Ramsey), who brandishes a shotgun, scaring them away. As they wander the streets, they encounter a motorcycle gang that picks on Tom, leading to a chaotic mishap where BB retaliates, grabbing the gang leader in a comical yet dangerous situation.
As a friendship blooms among Paul, Samantha, Tom, and BB, Halloween brings trouble. Samantha arrives at Paul’s home with a bloody nose, again seemingly victimized by her father. That night, in a reckless decision, the kids decide to prank Elvira. However, when BB accidentally reveals himself, Elvira shoots him, leaving Paul devastated while Tom grapples with guilt for initiating the prank.
During Thanksgiving, Paul and Jeannie share a meal with Samantha, leading to Paul and Samantha’s first kiss, establishing a deepening bond. Unfortunately, when Samantha returns home later that evening, her father brutally retaliates, leaving her gravely injured and in a coma. Paul learns from Dr. Johanson that her life will soon be terminated. Desperate, he recalls a novel idea: to harness BB's microchip technology to restore Samantha to life.
With Tom’s help, they procure keys to the hospital and navigate through it as Tom shuts off the power. When Dr. Johanson pulls the plug, darkness envelops the hospital, allowing Paul to sneak into Samantha's room to save her. He carefully inserts the microchip into her brain and activates it. To everyone's shock, Samantha stirs back to life, mechanically opening her eyes and breathing under Paul’s guidance.
Meanwhile, chaos ensues at Samantha’s home as her father discovers her absence. In a twist of fate, he stumbles into the cellar only to meet a horrific demise at the hands of his resurrected daughter. In a display of vengeance, Samantha eliminates him, leaving the scene shrouded in horror when Paul discovers her heinous act.
Samantha's power evolves as she takes revenge on Elvira as well, leading to an explosive confrontation that results in Elvira’s death in a scene full of gore and disbelief. When the police discover the bodies, Paul comes to terms with the growing monstrosity that Samantha has become, leading to an inevitable conflict between his best friend Tom and his own feelings towards Samantha.
As the climax unfolds, Samantha's chaos reaches a peak, leading to a tragic confrontation during which she attempts to defend Paul from the police's intervention. Despite her transformation toward humanity, she sacrifices herself to save him, leaving Paul heartbroken as he cradles her lifeless body in his arms.
In the chilling aftermath, Paul confronts the reality of loss in the morgue only to encounter a sinister entity embodied by BB emerging from Samantha’s body, resulting in a terrifying fate for Paul. The tale concludes with his screams echoing in the darkness as the credits roll, hauntingly repeating BB's name.
