Rural England, 1906. Idealistic missionary Charles Fortescue, newly returned to Britain after a decade in Africa, returns to Britain and is reunited with his fiancée Deborah Fitzbanks, who kept his letters in numbered boxes.

Fortescue is summoned to meet the sport-obsessed Bishop of London, who orders him to apply his experience to the prostitutes of London, in order to save their souls. The Bishop takes Fortescue to a seedy music-hall to show him the extent of the problem, which causes a fight to break out. Fortescue returns home to break the news to Deborah, but has difficulty explaining to his innocent fiancée exactly what "fallen women" do.

He visits Lord and Lady Fermleigh to try to get backing for his mission, but his lordship dies during his impassioned speech. He returns home to find that Deborah has independently contacted Lord Ames, the richest man in Britain. He goes to see them at an artist's studio, discovers that Lady Ames was the woman he met on the boat, and is disconcerted to find her professing her attraction for him. She invites him to her bolt-hole in Chelsea.

He returns home to tell Deborah that he has turned down the Ames' offer. She is appalled, as she thinks this will delay their marriage further. Reluctantly, Fortescue goes to see Lord and Lady Ames at their gigantic mansion, encountering their absent-minded butler Slatterthwaite. After the latter pours soup over Fortescue at dinner, Lady Ames insists that he stay the night. She visits him in his bedroom and tries to continue seducing him. After much protest, he eventually succumbs, and he receives the backing he needs.

Fortescue begins setting up the mission, and looks for prostitutes to save. The first is unimpressed by his plans, saying that she performs a service that she enjoys, and that she feels that the vicars who visit her are more hypocritical than she is. At night, he is visited by three prostitutes who have mistaken his stated views on the irrelevance of sex for a belief that what they do is not morally wrong.

Lady Ames visits Fortescue and discovers him in bed with three naked women. She also discovers that he and Deborah are due to be married. Appalled, she withdraws her funding. The prostitutes go back onto the street to raise money to rescue the mission.

Fortescue is visited by the Bishop of London, who accosts him with allegations that he is diverting prostitutes from the other missions by offering them sexual favours, and the Missionary Council has threatened to close down the mission and have Fortescue disciplined. The Bishop advises him that he might well achieve his own bishopric soon, and suggests that he hand the mission over to someone else.

Fortescue returns to the Ames' residence, but finds out that following an assassination attempt on her husband's life, they have moved to Scotland. Instead of attending his own wedding, he goes up there. Lady Ames is shocked to see him, especially after he reveals that he has worked out that she's the one who's been trying to kill her husband. During their conversation, she reveals that she is herself a former working-class prostitute. She flees the room, locking the door behind her.

While Fortescue tries to escape by scaling down the walls, Lady Ames drives to her husband's hunting party, where she plans to engineer an "accident" by bribing her Scottish servant Corbett (who is in love with her) to shoot him. Fortescue arrives at the crucial moment, pushing Lord Ames aside. The bullet hits Lady Ames instead, though she is only wounded. She concedes to Fortescue that she was wrong to try to convert a missionary. Fortescue is hailed as a hero by the rest of the party.

Deborah cancels the wedding and returns the presents. Lord Ames takes Slatterthwaite as his new bed companion. Fortescue returns to the mission, which becomes so successful that the Bishop of London closes it down in March 1907, offering Fortescue the choice of staying there or remaining in the Church. Fortescue chooses the latter, and continues to work with fallen women - alongside Lady Ames.