London on the eve of the Great War. Lord Adrian and Lady Drusilla St. Clair are unhappily married, without children. They live in some style, but a locked door separates their bedrooms. Their lack of passion is contrasted with the warmth of their dinner guests, cousin Pamela and her fiancé Stewart Sladen, a Scotland Yard detective. Later that evening Adrian and Drusilla come close to opening the door, but neither wants to be the first to do so.

When war is declared, Adrian is called up. In no man's land, sharing a cigarette with a cockney infantryman during an artillery bombardment, he says that war has woken him up. He returns to England convinced that he does love Drusilla after all; but she does not respond. Thinking of a stray remark made by the soldier, and reading in a book that a visit to 'darkest London' might feed his newly discovered appetite for life in the raw, Adrian thinks up a deception. He tells Drusilla he is going on a golfing trip, but instead dons workman's clothes and heads for the East End.

There he intervenes in a fight between local brute Herb Harris and a young woman, Vicky. Herb knocks both of them to the ground before seeing off Vicky's costermonger boyfriend Bill. Adrian, waking up in Vicky's bed, where she has been nursing him, poses as a local, 'Jim'. Vicky bathes in his presence, and there is clearly mutual attraction. Meanwhile, wondering about Adrian's secret life and worrying about their marriage, Drusilla unlocks the door.

Adrian broods in Vicky's flat. Though he rejects her advances, he is unwilling to leave her at Harris's mercy. He narrowly avoids being seen by Inspector Sladen, who is on patrol in the area, and, having devised a scheme to get Vicky and Bill out of their predicament, returns to Drusilla.

Sladen tells the St. Clairs that he is pursuing a notorious thief, thought by the police to be plotting murderous revenge on an unfortunate individual for coming between him and his girl. That same evening, alone with Adrian, Drusilla finally broaches the subject of their marriage, and they rediscover their mutual affection, realising each has misunderstood the other. Without explaining, however, Adrian sets off again into the London night, leaving Drusilla in tears.

Adrian tells Vicky and Bill his plan: they will hide away with a bagful of money that he has left in a railway station locker. However, Harris arrives at the flat. Adrian fights him, but Vicky has to step in and appears to kill him. Sladen, on patrol again, finds Adrian, who narrowly convinces him that he has been acting as a vigilante. By the time Sladen discovers Harris's body, Vicky has slipped away to retrieve the bag. Having seen Drusilla's photo in Adrian's wallet, however, she goes to their home. She faints on learning that he's married, and reassures Drusilla that he has been entirely faithful; and that she, not he, killed Harris.

Sladen arrives at the St. Clairs' and sees the bag Adrian told him he lost the cloak-room ticket for. Drusilla and Pamela, claim that Vicky has gone; in fact they have hidden her. When Adrian arrives, Sladen accuses him of killing Harris, but Adrian denies everything. After speaking to Scotland Yard on the telephone, Sladen lies that Vicky has already confessed to the murder, forcing Adrian to tell him that she did it in self-defence. Sladen advises him not to get embroiled in a murder trial. The St. Clairs hand Vicky over. When Adrian asks why she came to his home rather than escape, she says she wanted to see the woman who loved him.

While Sladen is on the phone a second time, telling his colleagues he has captured the suspect, Pamela helps Vicky get away. Adrian tells Sladen he will take the blame for it; but, as it turns out, Sladen has just been told that Harris is not, in fact, dead after all.