We open with a fight at Rockdale High School. A crowd of onlookers quickly gathers. Tony Rivers (Michael Landon) and Jimmy (Tony Marshall) are trading blows. As Jimmy seems to be gaining the upper hand, Tony throws dirt in his face, temporarily blinding him. The police arrive and the crowd quickly disperses. Detective Donovan (Barney Phillips) cleans Jimmy up, then has a chat with both. He makes them shake hands, and Jimmy walks off. Donovan tries to talk some sense into Tony, but he refuses help. This isn't the first scrape that Tony has had that involved the police. Donovan suggests professional help, but this only infuriates Tony. Tony saunters over to see his girl, Arlene (Yvonne Lime), who waited for him. He takes her home in his car.
Tony arrives home to find his Dad (Malcolm Atterbury) packing his lunch for work. Dad tries to convince his son that things would go better if he would just go along with people. That evening Tony arrives to pick up Arlene at her house. He honks the horn. This behavior reinforces for Arlene's parents, Bill Logan (S. John Launer) and Mary (Dorothy Crehan) that Tony is unsuitable for their daughter. They insist that Tony come in to the house to call on their daughter properly. They have a little chat with Tony, who is surly and petulant. Before they get in the car, Arlene insists that Tony see the doctor that Detective Donovan recommended--Dr. Brandon. Tony objects and refuses. The Halloween party is already under way at the clubhouse Tony was able to procure. We meet the rest of their clique. There's Frank (Michael Rougas) who arrived without a date, Vic (Ken Miller) the bongo player, and his girlfriend Pearl (Cynthia Chenault), who is dressed like a clown because she thought it was a costume party. A couple is making out on the couch, but he (James Best) gets a little too fresh with his date and gets his face slapped for his trouble. We are then treated to the vocal talents of Vic singing "Eene, Meeny, Miney, Mo". They exchange practical jokes, including the dead guy falling out of the closet gag, the bucket of water over the door jamb gag, and the stuff that jumps out of the candy box routine. When Vic blows a trumpet behind Tony, he attacks Vic and strikes Arlene when she tries to intercede. The assembled gang is shocked at this over reaction. At this point Tony realizes he must seek help.
Tony is being examined by Dr. Alfred Brandon (Whit Bissell). He explains that "before we enter the mind, we must know the condition of the body." His assistant, Dr. Hugo Wagner (Joseph Mell) is a reluctant assistant and provides Tony a mild sedative. Brandon tells Wagner that Tony is the perfect subject for his experiment. Through hypnosis Brandon will regress Tony back to a more primitive past and release savage instincts. He expresses no regrets that this will probably end in Tony's death. He injects the formula into his subject then hypnotizes him. Tony returns for another session, and each time Brandon questions him while he's under, taking him back further and further in time.
That evening at the clubhouse, Tony and Arlene decide to leave early. Frank, again dateless, refuses a ride with Tony and Arlene and decides to walk home, taking a short cut through the woods. Tony drops Arlene at home and drives away. Frank is walking through the woods and hears something. The rustling behind him is more noticeable and closer. He turns to confront it, but there is nothing there. He picks up the pace and then begins to run from the unseen menace. He then sees it and screams.
At the police station, a photographer hands officer Chris Stanley (Guy Williams) photos of Frank's body. Stanley brings the photos to Chief Pat Baker (Robert Griffin). Detective Donovan is in with the Chief discussing the case of Frank's murder. After examining the photos, Donovan suggests fangs were the murder weapon. Pepe the Janitor (Vladimir Sokoloff) posits his own theory to Officer Chris Stanley. It is something from the old country--a werewolf. The death makes the newspaper's front page.
Tony returns for another session. He suspects that something is wrong, and that he is somehow involved in Frank's death. The police continue their investigation. Tony meets with the principal (Louise Lewis) of his High School. She reviews Dr. Brandon's progress report and tells him he's doing very well. Tony exits the principal's office and heads down to the gym. He sees a gymnast practicing on the parallel bars and loiters to watch her. The school bell rings triggering a change in Tony. We see him transform into a werewolf. He attacks the gymnast, but she manages to scream for help and initially escapes his clutches. The commotion draws a crowd to the gym, including the principal from her office. He kills the girl and runs from the school and into the woods. The principal calls the police. When they arrive she tells them it was Tony. She recognized his jacket and trousers. Jimmy also confirms it was Tony.
The newspaper sends a journalist, Doyle (Eddie Marr) over to interview the Chief of Police. Detective Donovan goes over to interview Dr. Brandon about the werewolf. Brandon is skeptical about the reports, and due to doctor/patient confidentiality won't tell Donovan anything about Tony. The journalist then interviews Arlene and her parents, then later talks to Tony's father. The police organize a posse. They comb the woods but are not able to find Tony, now a werewolf, wandering around. He is aware the mob is looking for him. He is attacked by a German Shepard, but the dog gets the worst of the encounter. The posse gives up for the night, but Donovan reports they did find a dead dog, so he must be in the woods somewhere.
The next morning Tony awakes under a bush, but he has transformed back into human form. He heads out of the woods and into town. He spots a telephone booth at a gas station and calls Arlene at home. The police have staked out the house and want Arlene to keep him on the line long enough to trace the call. He never talks to Arlene and quickly leaves the phone booth and heads over to Dr. Brandon's office. He knows something is wrong and begs Brandon for help. He doesn't realize that Brandon is the cause of his transformations. Brandon wants to record on film the transformation and again engages Wagner in this final experiment. Brandon injects Tony and he transforms. The phone rings, triggering Tony's rampage. Tony first kills Wagner. He follows Brandon into his adjoining lab. He destroys the lab equipment before he kills Brandon. Donovan and Officer Stanley arrive and repeatedly shoot Tony. We see Tony lying motionless on the floor, but transformed back into his human form. We close with Donovan looking down on Tony's corpse and telling us that "it's not for man to interfere in the ways of God."