Gerry Carroll (Humphrey Bogart) is an artist on a fishing trip. He meets Sally (Barbara Stanwyck) and is taken with her beauty. While they are out fishing, he spends more time sketching her than fishing. She is swayed by his attention and soon they are in love. When a rainstorm drives them under a tree, Gerry removes his jacket and hands it to Sally so that he can retrieve his gear. An envelope falls from the jacket's pocket and, as Sally picks it up, she sees it is addressed to Gerry's wife. When she confronts him, Gerry admits he is married but quickly explains that his is a marriage in name only. His wife is an invalid and has been since the birth of their child. The letter is to ask his wife for a divorce, so that he and Sally can pursue their romance unburdened by his marriage. Sally storms off and Gerry returns home.
Gerry returns home, troubled by headaches, which are treated by a doctor. Soon, his wife dies and he is free to marry Sally. He begins work on a portrait of Sally, his new wife, but does not want her to see the portrait until it is finished.
One evening, Gerry's daughter is packing to go away to school. As Sally helps her pack, they begin to talk about her mother, Gerry's former wife. The daughter tells Sally how much her mother loved sports. She was even able to beat her father at tennis. Sally probes about her mother's illness and learns that Gerry's wife was not an invalid at all. Gerry has lied to her. The daughter tells her that Gerry's first wife died suddenly, shortly after Gerry had finished a portrait of her painted as the angel of death. Her death occurred shortly after Gerry returned from a fishing holiday, the same holiday where he met Sally. Sally and the daughter use Gerry's key to enter his locked studio. There they find the portrait of Sally, who has also been painted as the angel of death.
Meanwhile Gerry has been visiting a chemist, paying cash and purchasing a substance under an assumed name, Mr. Fleming. He returns home that evening to find Sally a bit distant and suspicious. He cannot get her to tell him what is wrong, though he is suspicious that someone has told her something to upset her. She claims to be tired and heads for bed. First, Gerry insists on bringing her a glass of milk, as is his habit every evening.
While Gerry is distracted by the phone, Sally dumps the milk out the window, fearing it contains poison. She retires to her bedroom and locks the door. Downstairs, Gerry sees where some of the milk has been spilled by the window. He realizes Sally has not drunk the tainted milk; she must know his plan. Trying to decide what to do, he glimpses a headline about a murderer who breaks in homes and strangles his victims. Gerry becomes frantic, stages a break in, trashes the room, and goes to the bedroom to find that Sally has locked him out. He cannot convince her to open the door.
Sally is able to make a quick phone call just before Gerry pulls the telephone wire. He enters the bedroom from the window and, during their confrontation, his intention is plain. He must kill Sally; he is driven by madness. The police arrive in the nick of time and take Gerry off to jail, but not before he offers them a glass of milk.