Onboard a ship traveling to Liverpool, England from the West Indies, Anglican missionary's widow Olivia Harwood is prevailed on to help nurse malarial patients on the lower decks. There she meets the suavely handsome Mark Bellis, who has been taken ill. Despite Mark's vagueness about his life and past, the couple strike up a friendship. Fully recovered by the time the ship docks, Mark persuades Olivia to allow him to take up residence in the house she has inherited from her late husband, where she takes in lodgers. He proceeds to work a smooth line of seduction on her, while still finding time to also use his charms on the more worldly and vulgar Kitty, who he had a relationship with before he fled to Jamaica.Mark's past as an art thief and forger is revealed as he reunites with former partner-in-crime Edgar Bellamy and the two plan a daring art heist. Things go awry, and they are forced into a rooftop flight, narrowly avoiding police bullets. Returning to Olivia, he tells her he intends to leave London to try to make good elsewhere. However, she has now fallen under his romantic spell and is prepared to do anything to keep him with her. The couple are in dire need of money, and Olivia is persuaded to insinuate herself into the home of her wealthy former school friend Susan Courtney and her older husband, Henry. She finds Susan in a state of neurosis and barely suppressed hysteria, worn down by the criticisms of the cold and sneering Henry, who agrees to employ her as Susan's live-in companion. Under Mark's urging, she immediately begins to pilfer stocks and bonds and small valuables from the Courtney household, passing them on to Mark to turn into cash.Mark meanwhile has discovered an old bundle of letters from Susan to Olivia, containing youthfully indiscreet descriptions of romantic dalliances and questionable moral conduct. Realising that making public the contents of the letters would ruin the Courtneys' social reputation, he believes that he has hit the financial jackpot. As low as she has already sunk under his influence however, Olivia finds the notion of blackmail repugnant and a step too far down the road of criminality. She flees from the Courtneys and looks into the possibility of a return to overseas missionary work, only to find that a lone woman is not wanted. She finds herself sheltering in a gloomy church, where Mark somehow manages to track her down. In despair, she falls for his blandishments and submits herself again to his control and instructions, blackmail and all.Olivia returns to the Courtney household and sets in motion the blackmail plan, while Mark continues to dally with Kitty and gifts her a locket which was given to him by Olivia. Unknown to Olivia or Susan, Henry has become exasperated by Susan's apparent inability to produce the heir he craves, and is plotting to have her committed to a distant mental asylum. He has also employed a private detective, who has managed to trace the missing stocks and bonds back to Mark and has built up a dossier of his criminal past.Henry locks the horrified Susan in her room to await the arrival of the sanatorium doctors and orders Olivia out of the house. At Mark's behest, she returns to step up the blackmail threat, but is countered by Henry confronting her with the information he has on Mark, which would be more than enough to hang him. A struggle ensues, and Henry collapses with a life-threatening heart attack. Olivia releases Susan and tricks her into giving her husband a dose of medicine laced with poison. Henry succumbs, the police are summoned and the hopelessly confused and incoherent Susan makes what sounds like a confession to murder. She is taken away to prison to face the prospect of the gallows.Mark announces his intention to take Olivia away with him to a new life in America, beyond the reach of British prosecution. Olivia however is conscience-stricken about Susan, and matters take a fatal turn when she runs into Kitty, wearing the incriminating locket. All her illusions about Mark's love for her suddenly shattered, she finally realizes that she has all along been no more than a willing pawn in his game. Keeping her own counsel, she waits until the opportunity arises in a hansom cab to take her ultimate revenge by fatally stabbing Mark. The film ends with Olivia entering a police station to turn herself in.