Kindergarten Cop is action comedy movie about a undercover detective taking on the hardest job in the world, a kindergarten teacher. John Kimble(Arnold Schwarzenegger) and his partner Phoebe O'Hara(Pamela Reed)receive a viable tip that Cullen Crisp(Richard Tyson)a very dangerous man has just had someone kill one of the informants who was planning on testifying against him. Before killing the informant, Crisp got the information where his son, Dominic and ex-wife, Rachel is hiding.
O'Hara agrees to go undercover as she has elementary teacher experience at the school where Crisp's ex-wife and son are. The only catch is they do not know who the ex-wife and son are, just where they live. At last minute, O'Hara gets sick and Kimble has to take her place as the substitute teacher. Kimble has absolutely no experience in handling kids.
At first, the classroom is a zoo, the kids do not listen to him and they run the classroom. With the help of his pet ferret and strict police training, Kimble quickly whips the kids into shape. The principal who at first did not trust Kimble, but when an incident of child abuse arises and Kimble takes care of it; he wins her over.
While trying to find out who Rachel is, Kimble befriends one of the teachers at the school. In addition to being a teacher, Joyce Palmieri(Penelope Ann Miller)is also the mother of one of his students. The more Kimble and Palmieri get to know each other the more trusting she becomes, the more she tells Kimble of her double life; she is really Rachel Crisp. There was no money stolen and she went into hiding because how dangerous Crisp is.
Crisp is let out of jail and he and his mother, Eleanor Crisp(Carroll Baker) immediately head to where Rachel and Dominic are living. There is a fire started in the Library to cause a distraction while Crisp tries to take his son. Realizing that Dominic is not with the rest of the class, Kimble goes back and is confronted with Crisp taking his own son hostage. Kimble is shot by Crisp, but Kimble kills Crisp.
In the meantime, Eleanor tries to run over O'Hara. She enters the school and finds her son shot dead and Kimble wounded. She shoots him in the shoulder. She raises the gun again to kill him, but is knocked out by O'Hara with a baseball bat.
After the incident, Kimble makes the decision to retire and goes back teaching.