Based on a novel by Dashiell Hammett, a retired detective and his wife cleverly solve three murders that have sent the police astray.
Dorothy (Maureen O'Sullivan) tells her father Wynant (Edward Ellis) she is getting married. Inventor Wynant gets $1,000 from his lawyer MacCauley (Porter Hall) and tells him he is going away. Wynant goes to his secretary Julia Wolfe to get missing bonds worth $50,000 to give his daughter Dorothy as a wedding present; he accuses her of taking them and is also angry at her, because he found her with Joe Morelli (Edward Brophy). Dorothy tells Nick Charles (William Powell) her father has disappeared. Nick and his wife Nora (Myrna Loy) drink six martinis each. Dorothy's mother Mimi (Minna Gombell) needs money, because Chris Jorgenson (César Romero) doesn't work. The body of Julia Wolfe is found murdered. Police detective Guild (Nat Pendleton) questions Mimi. Mimi took Wynant's watch-chain from Julia's dead hand; she shows it to Dorothy and hides it in a safe. At the Charles' cocktail party Dorothy pulls a gun on Nick and says she killed Julia; but Nick's questions show she is lying.
Late Christmas Eve Joe calls on Nick and pulls a gun. As the police arrive, Nick wrestles Joe for his gun; Joe is arrested. Guild finds a gun in Nora's bedroom drawer. Nick gets a wire from Wynant asking him to take the case. Nick and Guild call on Nunheim (Harold Huber) as Marion leaves. Nunheim runs out the back but is shot later on the street. Mimi gives Guild Wynant's watch-chain. Nick goes to Wynant's shop with his dog Afta and a flashlight; he reports to Guild a dead body. Wynant's former employee comes in, and Nick questions him. Police discover the body was dead two months or more, and clothes indicate a big man with the initial R. Guild suspects Wynant committed all three murders.
Nick tells Nora the body is Wynant. He has Guild invite the suspects to a dinner party, hiring bodyguards as waiters. Nick announces Wynant is dead and says the murderer is at the table. Nick explains Wynant looked for the man cheating with Julia. This man killed Wynant, Julia, and Nunheim. Since Mimi said she saw Wynant, Nick asks her who made her say that. As MacCauley pulls a gun on Mimi under the table, Nick slugs him and pronounces him the murderer. In the final scene Nick and Nora are on the train back to California accompanied by Dorothy and Tommy on their honeymoon.
Buoyed by a happily married couple's witty banter, this charming film started a series and even a trend. Even the dog provides laughs. The constant drinking seems to be a celebration of Prohibition ending.