Into the Pool of London on the River Thames sails the Dunbar, a cargo ship out of Rotterdam, and the seamen on board disembark for a weekend of leave.
Dan MacDonald (Bonar Colleano) and his best friend, Jamaican Johnny Lambert (Earl Cameron) meet up with Dan’s girlfriend Maisie (Moira Lister) while Johnny befriends ticket seller Pat (Susan Shaw).
Dan supplements his wages with some light smuggling of cigarettes, nylons, bottles of brandy and the like. As he drops some cigarettes off for one of his contacts, he is approached by a stranger who offers him £100 to smuggle a small box to Rotterdam.
It transpires that the stranger performed a jewel heist with his gang but the robbery went wrong and they killed the night watchman. With Scotland Yard on the trail of the jewels, Dan asks Johnny to carry the box on board the Dunbar.
Maisie’s sister blabs to the authorities and the ship is held in port by the police and custom’s officers who are waiting for Dan to appear so they can arrest him.
Meanwhile, the robbery mob are after Dan also. Shoved into a car at gunpoint, they demand he returns the diamonds – which he can’t do since he already gave them to Johnny.
He is shot in the shoulder but escapes and heads for the Dunbar to stop his friend from boarding the ship and being caught by the police. Arriving just in time, Dan collects the package from Johnny and turns himself in to the police.
Engine Room Officer Trotter
James Robertson Justice
Customs Officer Andrews
Det. Insp. Williams