As a child, Charlie Underhill is at a playground when bullies pick on him.
In the present, Charles plays with his son Steve. They're visiting Carol, Charles' sister, who notes that Steve needs friends and a mother since Charles' wife Marion died. Carol points out that she's getting married and points out that Charlie has never taken his son to the playground nearby. Charles refuses to take him there but Carol tells her brother that he's never grown up and never adapted. Charles reluctantly agrees to check out the playground. Carol insists that he take Steve there that night and Steve picks up on it. As he prepares to go to work, Charles hears children singing outside the house.
At the playground, Charles finds children playing normally, going down the slide and swinging. However, he soon sees them attacking each other and another young girl trips. Charles stares in horror as the children tear at each other, bleeding and bruised. Finally they gang up on a young boy and Charles tells them to stop. The children run off, leaving their last victim behind. The boy stares at Charles and then leaves without saying a word. Charles turns and leaves, unaware that a boy is watching him.
Over supper, Charles insists that there is something wrong with the playground. Carol thinks that he's imagining things and points out he hasn't been to a playground in 20 years. When Steve asks to go, Charles doesn't say anything.
Later, Charles takes his son to the playground but holds him back. All of the children grow silent and watch the newcomers for a few seconds and then start playing again. Charles has Steve stay back and he goes in to check things out. The children grow silent again, watching him. A boy atop the slide calls to Charles by name and tells him to come in and play. The other children turn and glare at Charles as the boy in the shadows tells him that he's it. Charles backs away in terror and tries to close the gate, but the children tear at him, transformed into savage beasts. They rip at his coat and then revert to normal, chanting his name. As Charles takes his son away, the boy on the slide tells him to come back soon, goes down the slide… and disappears.
At home, Charles tells Carol what happened and refuse to let Steve go there. Carol insists that Steve has to learn how to survive in the real world, but Charles wants to stand in for his son forever to protect him. Carol points out that he survived, and Charles says he barely did. As they talk, Steve listens from upstairs.
As Charles rides to work on the subway with his friend Bob, he wonders how to raise a boy but his friend isn't interested and talks about work. Later, Charles walks home and discovers that Carol has taken Steve to the playground. Steve runs into his father's arms and asks if they can come back tomorrow. As they go, the boy on the slide calls out to Charles, who realizes it's Ralph, a childhood bully. Charles explains to Carol that Ralph used to wait for him on the corner every day. Ralph dares him to come in and then vanishes down the slide again. Charles runs away in terror.
Later, Charles tucks Steve into bed and contemplates a photo of his wife. He remembers how Ralph and the other children picked on him as a boy. They would chase him back to his home where he would pound on the door, begging to be let in. Downstairs, Carol asks her brother what the matter is at 3 a.m. and he talks about how he used to run home to find some place to hide, but their parents were at work and the front door was locked, leaving him trapped. Charlie ran to the playground to find a place to escape, and all he could do was go inside his own mind. Now he can't stand to do nothing while Steve suffers what he believes will be the same fate.
Charles goes to the playground and finds it abandoned. He looks around and briefly hears the sound of children pounding and screaming. Charles goes to the slide and calls out to Ralph. He tells the bully that he'll bring Steve there and he's to leave him alone. Ralph steps down the stairs with a stick and advances on Charles. He throws the stick, shattering the street light outside, and then disappears. Charles runs away.
The next day, Charlie tells Steve to do exactly what he says as they go the playground. Steve says that he's brave and will be the daddy. As they go to the playground, Steve keeps chanting that he's the daddy. They arrive and all the children turn to look. Charles brings his son in, and Steve asks who he is, and then insists that Charles is Steve. Charles accepts that he is and climbs the ladder to the top of the slide. Ralph is waiting for him at the top but Charles climbs up, refusing to yield. Ralph goes down the slide and Steve shoves Charles down and then follows after him.
At the bottom, Charles discovers that he has transformed into Steve, and Ralph is waiting for him. Ralph and the other children chase after Charles and trap him inside the jungle gym, Steve in his father's body smiles and then starts swinging. Finally he walks away from the crying Charles, waving goodbye as the children begin to transform into monsters.