Harry Hawkins doesn't like his new step-father Bill Wesley, nor his bossy step grandmother, who constantly finds fault with him. His mother seems to have forgotten his father, a pilot who died in the war, in her new marriage and new baby. Being bullied at school, picked on by teachers, Harry just feels he has no place to go. The day he finds Ocky, an escaped chimpanzee from the circus he decides to look after her in his secret den on the bombsite, glad to have a friend at last. But an unexploded bomb is discovered, and with it, Harry's den and his hiding place for Ocky. Harry trusts Father Murphy with his secret, but Father Murphy passes on the information to the other adults, and Harry and Ocky are forced to run. After a roadside adventure with gypsies, Harry makes his way with Ocky to Aunty Ivy, the owner of a sea-side boarding house where he had his last holiday, where he last felt happy. Ivy too betrays him, calling his parents and the police, who catch up with Harry. To escape their pursuers, Ocky jumps off the pier, and Harry after her, and as Bill jumps in after his step-son, Harry realises how much he's cared for. Back at home, Harry knows he has to return Ocky to the circus and finds her owner needs her more than he realised - he's a blind clown called Mr Nobody, who can only do his act with Ocky as his eyes.