Sibling rivalry between eleven-year-old Terry and his thirteen-year-old sister Tracey causes disharmony in the decent, hardworking Harmer family. Mum wants Terry's new shirt kept good for his birthday party that Saturday but he sneakily wears it to school, drawing approving comments. When Tracey tells mum, a row erupts. Tracey says Terry's his mummy's baby and he snaps back, "No, I'm bloody not!" Mum slaps Terry for his language and he runs away from home.

Terry shelters from a storm in the old park bandstand, where four toughs accost him. From Napier school, they don't like Terry's Fox Hill school. Terry makes a break for it but is caught and frogmarched through London's rundown docklands to the hideout of ringleader Les. Les wants to break into Fox Hill and, threatening Terry with violence, demands to be led to its collection of expensive radios.

The gang and Terry break in and, with Terry's inside information, easily find the radios, but are disturbed by the caretaker. Escaping through a window, the gang leap the school's locked fence. Les and Terry are last over, Les pulling Terry to safety. Les has stolen two radios and gives one to Terry 'for insurance', demanding he give it back to Les tomorrow.

Next day, police investigate the break-in. The caretaker's description of a boy in jeans and a black shirt points to Terry. Shamed by his classmates, Terry owns up to the Headmaster. He initially believes Terry's story but when the boy returns his radio from hiding, the Head's mood has changed - the caretaker now recalls hearing Les call Terry by the name "Tel", suggesting they were friends and accomplices. If the second radio isn't returned Terry will face charges.

Tracing Les's address, Terry asks for the second radio back or he'll turn the gang in. They are interrupted by Les's mum, who hits her son for messing up the carpet - maybe Les isn't such a hard man after all. Les sold the radio to a dealer that morning but agrees to steal it back for Terry. They retrieve the radio but a policeman catches Terry red-handed with it, climbing the dealer's fence. The officer asks if anyone is hiding over the fence. Seeing Les gesturing as if pleading not to be turned in, Terry denies it but the knowing officer arrests them both.

Given background reports, the juvenile court is willing to let Terry go but wonders why he covered for Les when on the fence - was Terry hiding his accomplice? When he explains that he felt sorry for Les, the court is satisfied and discharges Terry.