London docks, Friday afternoon. Merchant seaman Johnny, who is black, and his friend Dan stop by a local theatre. While Dan speaks to actor Vernon, Johnny watches the show through a partly open door. A doorman appears and racially abuses Johnny, causing Pat, the ticket seller, comes to his aid. They strike up an immediate rapport. Dan reappears and scolds the doorman for being racist, administering a couple of slaps for good measure. Johnny and Pat agree to meet the next day.

Saturday. Aboard ship, Dan gives Johnny pairs of nylons. As they are leaving the ship, Dan is searched by customs and his nylons are confiscated but Johnny is unhindered. Once ashore, Johnny and Pat go dancing with Dan and his girlfriend Maisie. Maisie, peeved that Dan has no nylons for her, insults Johnny and remarks that Pat must be really hard up to take nylons from him. Johnny overhears and leaves the club in disgust. When Pat catches up with him, Johnny tells her he is fed up with places like London and intends to go back home to Jamaica.

Vernon and his gang make final plans to steal a set of diamonds. Dan sleeps with absent shipmate Harry's girlfriend.

Sunday. Before going ashore Dan, fearing he's now marked by customs, asks Johnny to smuggle a package back aboard ship for him. Johnny agrees without hesitation.

The time of the robbery arrives. Vernon breaks into the building, knocks the guard unconscious and lets in the others. Meanwhile Johnny and Pat visit the National Maritime Museum.

Vernon and his gang crack the safe and take the diamonds. Police arrive as the gang are making their escape. A car chase ensues. The gang's car goes over a jetty and ends up in the river. Vernon manages to escape with the diamonds.

Vernon meets Dan in a church and gives him the diamonds to take to Amsterdam, his ship's next port of call. Later, Dan learns that the guard has died from his injuries. Filled with remorse, he wants to ditch the diamonds but Maisie persuades him to give them to Johnny. Masie's sister overhears their conversation and is later picked up by the police. Dan meets up with Johnny and gives him a sealed package to smuggle on board the ship. Johnny is unaware that the package contains diamonds.

The police, who are now looking for Dan persuade the captain not to sail until the morning tide. Realising the danger he is in, Dan tries to find Johnny to retrieve the diamonds, with the police and Vernon's gang in pursuit.

Johnny gets to Pat's house but at the last moment gets cold feet and walks away. Vernon and his gang find Dan and bundle him into a car at gunpoint. The police spot them and give chase. Dan tells Vernon he hasn't got the diamonds but Vernon doesn't believe him. There is a scuffle and Dan jumps from the moving car, but as he is running away Vernon shoots him and he falls into the river. The police chase Vernon and his gang and eventually they are all caught except Vernon, who is chased up a tall building, from which he falls to his death.

Dan crawls out of the river wounded and hitches a lift back to the docks. Johnny wakes up in the street, after being plied with drink by a few locals and robbed (though not of the diamonds). Dan appears and asks Johnny for the diamonds, which he hands over. Fearing Johnny will be caught with him, Dan racially abuses Johnny to be rid of him quickly. Puzzled, Johnny boards the ship and Dan gives himself up to the police. The ship sails.