High school student Vernon Potts rides his bicycle to English class, where he and fellow students watch a film version of Dr. Jekyll and Mr. Hyde. As the teacher, Miss Grindstaff, dismisses the class, she calls Vernon to her desk and reprimands him for turning in a biology report instead of his assigned paper on author Robert Louis Stevenson. Although Vernon finds the English class essay in his briefcase, Miss Grindstaff destroys his prized biology report with a paper cutter. In the science laboratory, Vernon shoos away a cat chasing his white guinea pig, Mr. Mumps, but the menacing janitor, Mr. Griggs, warns the boy to leave his pet cat alone. As Vernon whispers to Mr. Mumps that his experimental formula will make him famous, a pretty classmate named Robin Jones comes in and consoles him for Miss Grindstaff’s behavior. Just then, Robin’s boyfriend, Roger Davis, drags her off and warns Vernon to stay away from her. Sometime later, Vernon asks Coach McCall for a pass so he can work on his science experiment during gym class, explaining that he is on the verge of discovering of an important formula that can alter the physical makeup of human beings. When the coach refuses, Vernon is accosted by Roger and his friends, who destroy the notebook containing Vernon’s research. That night, Vernon breaks into the lab and discovers that his special formula has been a success: Mr. Mumps has transformed into an aggressive, black creature that has killed Mr. Griggs’s cat. As the janitor calls his pet, Vernon dims the lights and believes he has outwitted the man, but Griggs returns, accuses the boy of murder and beats him with a flashlight. Griggs forces Vernon to drink the formula, then bludgeons Mr. Mumps and goes after the boy. Cowering in pain, Vernon transforms into a killer and plunges the janitor’s face into a vat of sulfuric acid. Awakening at home, covered in blood, Vernon realizes he is the victim of his own foolhardy experiment. He washes his clothes and returns to the lab, where he dumps his laundry bag into the vat of sulfuric acid. As Vernon scours the room, science teacher Mr. Henshaw arrives and the boy reports a break in. During class, a girl screams upon finding Griggs’s skull in the acid vat and when Lieutenant Bozeman and two police officers arrive to investigate, they notice a bruise on Vernon’s face. Sometime later, Robin apologizes to Vernon about her boyfriend Roger’s attack, but she is interrupted by Miss Grindstaff, who warns the boy that he is failing English; the teacher orders Vernon to attend Literature Club meetings to ensure his graduation. That night before the club meeting, Vernon drinks his potion, transforms into a killer and decapitates Miss Grindstaff with her paper cutter. Lieutenant Bozeman returns to the school and asks Vernon about his guinea pig, then reveals that the rodent’s bones were discovered in the sulfuric acid along with those of Mr. Griggs and his cat. When the lieutenant leaves, Vernon begins to transform into his malicious alter ego, but Robin cools him with a towel. Consoling the boy over the loss of Mr. Mumps, Robin asks Vernon to help her with her science project that evening. When Vernon arrives late to gym class, Coach McCall offers to give him a pass for the rest of the year on the condition that he supply the star football player, Roger Davis, the correct answers for an upcoming chemistry exam. Vernon and the coach arrange to meet that evening in the gym. Later, at Robin’s house, Vernon explains that Homo sapiens were set apart from their ancestors because they had the capacity to choose between good and evil; Robin kisses him, but he runs away to meet Coach McCall. Waiting in the gym, the coach hears a noise and calls Vernon’s name, but he is accosted by Bozeman and two police officers who are looking for suspects. When McCall defends his innocence and the men leave, Vernon drinks his potion from a beaker, transforms into a killer, and stomps McCall to death with soccer cleats. Hearing the commotion, Bozeman rushes back into the gym and finds his prime suspect, McCall, swinging from the rafters as his fellow officers discover Roger outside. The next day, Bozeman pulls his police car alongside Vernon and reports that Roger has been arrested for the three murders. Noticing a laundry bag attached to Vernon’s bicycle, Bozeman claims his sister works at the cleaners and offers to drop off the items, but Vernon refuses. Wielding a lab beaker that was found near McCall’s corpse, Bozeman reports that the coach called Vernon’s name before he was killed and Vernon quickly rides away to school. There, Robin storms into the lab to announce that her boyfriend may be a killer. When Vernon proposes they go to the symphony that weekend, Robin comments that his personality has changed. Later, as Vernon prepares for the date, he receives a phone call from Roger, who reports that he was released from prison due to lack of evidence. Roger claims he saw someone break into the lab on the night of McCall’s murder and Vernon agrees to meet with Roger at school. Arriving at the lab early to concoct his potion, Vernon is frightened by the sound of an intruder and discovers Robin, who warns that Roger has set a trap. As Vernon contorts with anger, he confesses to the murders, but Robin does not believe him. He insists that the victims deserved their fate, transforms into his alter ego, and chases Robin through the halls. The terrified girl is unable to escape, but Vernon cannot kill her. As Bozeman and his officers break into the school, Vernon runs to find Roger. He attacks the boy but a policeman shoots Vernon. Mourning her lost love, Robin kneels beside Vernon’s dead body.