Phillip Davidson (John Mills) boards a boat and embraces Fay Driver (Elizabeth Sellars). Then he goes down below to try to convince her father, Captain Driver, not to involve Fay in his criminal activity. However, Boyd (John Chandos) brings aboard Delaney (a man he has agreed to smuggle out of the country) and two henchmen. When Boyd demands that Delaney pay him £500, rather than £200, a fight erupts, and Boyd knocks Delaney out. A broken oil lamp starts a fire, attracting the attention of the authorities, and Davidson is fished out of the water. A charred corpse is found in the sunken boat. The Drivers and Tim Pewsey perjure themselves by identifying the dead man as Boyd, rather than Delaney, and claiming there was no other man present. This leads to Davidson's conviction for Boyd's murder. Granted leniency, he spends 12 years in prison.
Upon his release, he sets out to get even with the witnesses. He is kept under surveillance by the police on the orders of Superintendent Bob Lowther (John McCallum), who is now married to Fay. Davidson finds an abandoned barge claimed by Jackson, a kindly old hermit. His plan is to live rough on the barge while he searches for the witnesses. But three people attempt – initially unsuccessfully – to befriend him. First Jackson withdraws an initial request for rent. Then Craig, a newspaperman who suspects him to be innocent, arrives; Davidson throws him out, but Craig tumbles down an open hatch and is knocked unconscious, and Davidson rescues him. Finally he happens upon a sailor attempting to rape Ilse, a traumatised wartime refugee; when he rescues her and allows her to stay on the barge, she falls in love with him.
Informed by Craig that Captain Driver had died four years earlier, Davidson stalks Pewsey, with Lowther and Craig on his trail. Pewsey is frightened into confessing to Lowther that there was another man present at the murder. Now Lowther's marriage comes under increasing tension as he considers the possibility of his wife's perjury. Finally, she confesses she did lie to protect her father. Lowther tells her that she will have to turn herself in and he will have to resign. She asks for time, and goes to see "George Barry", who turns out to be Boyd. She asks him for money and they plan to leave the country together.
Ilse pleads with Davidson to give up his dream of revenge and start a new life with her. He confronts Fay in her home, realizes Ilse was right, and walks away. However, by sheer chance, he is then hired to deliver an urgent letter to "Barry". Davidson confronts Boyd in his office, they fight, and then Davidson stops fighting and walks away again. It is time for Boyd to meet Fay at London Waterloo station, but he pursues Davidson and shoots him in the arm.
When Fay realizes Boyd is not coming, she attempts suicide by jumping in front of an oncoming tube train, but is stopped. She leaves with police sent by her husband after he read her farewell note.
Davidson flees to the barge, but Boyd is waiting for him. After a chase, Boyd is about to kill Davidson when he is shot dead by Jackson. Ilse and Davidson refuse further help from the police, leaving to deal with their pasts together.
The film was made at Pinewood Studios and on location in Kent around Gravesend and at Shad Thames in London. Many of the houses shown in the film were demolished soon afterwards. It was the last film of Henry Edwards, a major British star of the 1920s and 1930s, who had a small role as a judge early in the film.
The film had its gala premiere at the Leicester Square Theatre on 22 January 1953, with Prince Henry, Duke of Gloucester and Princess Alice, Duchess of Gloucester as guests of honour, and went on general release the following day. The Times film reviewer found the film a bit dull and self-important, but gave director Hamer credit for "effective use of the film's natural background, the mud and desolation of the flats of the Thames Estuary."