A young boy, known as Scimpy, lives with his thieving grandmother and gambling rag and bone man grandfather. He believes his mother to be away on holiday and his father at sea, until he overhears his grandmother talking about them. He learns that his mother, Rosie, and his father, known as Ricketts, are both in prison.

He sets out to visit his mother, asking a policeman in the street how to get to prison. The policeman explains that prisons are only for bad people and instructs him to go home. Instead, he steals a charity collection box and presents this to the policeman.

At the police station, Inspector Brisby is sympathetic to Scimpy's plight and says he'll arrange a prison visit. Scimpy is sent to stay the night at a remand home. Falling in with another boy at the home, he escapes and that night the pair sneak into Harrods, where they play in the toy section.

In prison, Ricketts is summoned to the governor's office and told about his son's predicament. He also learns that his wife is to be released the next day. The governor grants him a day's release to make arrangements for Scimpy's future.

Upon release, Rosie and friends go drinking in Covent Garden. A friend tells her about Scimpy's remand. Ricketts goes to the Magpie and Goat pub in Notting Hill, where he is greeted by his friends and fellow criminals.

Scimpy reaches Holloway prison and is told of his mother's release.

Rosie also makes her way to the Magpie and Goat, where she is reunited with Ricketts and a party is held to celebrate their release. At the police station, an informant maliciously claims that Big Al, a friend of Ricketts' who had earlier thrown the informant out of the pub, had used a weapon against him and is planning on attacking another man to provoke a gang war. Inspector Brisby doesn't believe him, but agrees to investigate. He also learns of Scimpy's escape from the remand home.

With the party in the pub in full swing, Inspector Brisby arrives to see Big Al, who agrees to go the station for questioning afterwards. Scimpy arrives and is reunited with his mother. Brisby tries to intervene, to return Scimpy to state care. A brawl results and uniformed police pile in. Scimpy is ushered out by one of Rosie's friends in the confusion.

Ricketts, Rosie and many of their friends from the pub are held by police. Rosie is sent home and Ricketts back to prison. Most of the others are allowed to leave but Inspector Brisby informs them they must let him know if they see Scimpy, who he is looking out for.

Later, Scimpy is playing in a park, accompanied by Rosie's friend. A policeman spots them. He explains that they must be taken in and Scimpy will be looked after by the state.

In prison, Ricketts receives a letter informing him that Rosie has been convicted again and sentenced to two years, just as he is about to be released. Scimpy is in state care.

In Holloway, Rosie is reunited with old prison friends and reports that she's now pregnant. Scimpy had been born in prison and now her second child will be too.

Social workers discuss Scimpy's case and decide it's best to send him back to the care of his grandparents for rehabilitation at home. First, however, he is sent to a reception centre, from which he escapes and hitches a lift to London, intent on visiting his mother in prison.