During post-war England, a young petty crook, Jackie Knowles, is on probation for committing petty crimes. Jackie wants to marry his girlfriend Kitty but they don't have enough money to live on. Jackie foolishly accepts to be the getaway driver for a robbery job. The robbery goes wrong and Jackie is arrested. Because of his previous probation breach, the judge sentences him to three years in a Borstal institution. In the UK, the Borstal institutions are trying to rehabilitate rather than punish young offenders. Jackie Knowles arrives at a Borstal institution run by a sympathetic governor. Initially, Jackie intends to keep his nose clean, do his time and get out. Unfortunatelly, he falls in with a crowd planning to escape. The wrong crowd is run by scheming Welsh boy Alfie Rawlins who senses Jackie's naiveté and innocence. Alfie manipulates Jackie into wanting to join his group by playing on Jackie's anxiety about his girlfriend, Kitty. Alfie lies that Kitty already has forgotten about...

The attack by British midget submarines on the German battleship "Tirpitz" in a Norwegian fjord during the Second World War.