Married sleuths Nick and Nora Charles try to clear Nora's cousin of a murder charge.

On New Year's Eve, Nick and Nora Charles arrive back home in San Francisco after Nick successfully solved the "Thin Man" murder case in New York. Exhausted from their trip, Nick and Nora want a quiet evening at home, but discover that their house has been taken over by a group of revelers they don't even know. When Nora's Great Aunt Katherine Forrest invites them to dinner, Nick doesn't want to go, but agrees when Nora's cousin, Selma Landis, pleads with Nora. At Katherine's house, they discover that Selma's husband Robert has been missing for three days. Katherine asks Nick to quietly find Robert, and, although he is reluctant, he takes Nora to the Lichee, a Chinese nightclub that Robert frequents. Robert has been at the Lichee, drinking and waiting for the club's entertainer, Polly Byrnes. Polly and "Dancer," one of the club's owners, are expecting to get money from Robert and Dancer tells Polly to take Robert home. A few days previously, Robert had approached David Graham, Selma's former fiancée, for $25,000 to leave for good. David meets Robert a short time later to give him negotiable bonds, then, after cruelly bidding Selma goodbye, Robert walks out into the fog and is shot. When David's car drives up to Robert's body, Selma is standing over it with a gun. Dazed, Selma gives David the weapon and he tells her not to say anything. Despite their efforts to protect Selma, Nick and Nora are unable to prevent her arrest by Lieutenant Abrams. Though Selma says that she had not fired her gun, David reveals that he threw it into the bay, thinking that Selma actually had killed her husband. The next day, Nick goes to the hotel room of Phil Byrnes, a man posing as Polly's brother, but actually her husband. Upon his arrival, Nick discovers Phil's dead body. Later, he also discovers that someone had been listening to Polly's apartment through a device in the apartment above. Suspicious when he hears Dancer enter Polly's apartment, Nick follows him to the basement and finds the body of the janitor. When Nora arrives at the apartment building and hears the janitor's name, Pedro, she reveals that Pedro used to be her father's gardener. Nick then asks Abrams to have all of the suspects congregate at Polly's. Though Dancer and Polly admit their plan to use a check forged with Robert's name, each claims to be innocent of the murders. During questioning, David says that he remembers Pedro, a man with a long white moustache, but hasn't seen him recently. When Nick looks at a picture of Pedro taken years before and sees that Pedro then had a small black moustache, he knows that David must be lying. Nick then says that the murderer has finally made a slip and reconstructs the evidence to reveal that David killed Robert out of revenge, then killed Phil when Phil tried to blackmail him. Finally, when Pedro recognized him, David was forced to kill him as well. Now cornered, David reveals that he had been planning to frame Selma for her husbands murder. He draws a gun and threatens to shoot Selma and then himself, but Lum Kee, Dancer's partner in the club, knocks the gun out of David's hand and David is overpowered. A short time later, Nick and Nora leave San Francisco on the train, accompanied by Selma, who plans to start a new life. Finally, when he is alone with Nora, Nick sees that she is crocheting a baby's sock and is shocked when she says "And you call yourself a detective."