The young Gascon D'Artagnan arrives in Paris, his heart set on joining the king's Musketeers. He is taken under the wings of three of the most respected and feared Musketeers, Porthos, Aramis, and Athos. Together they fight to save France and the honor of a lady from the machinations of the powerful Cardinal Richelieu.

In 1938, Walter Neff, an experienced salesman of the Pacific All Risk Insurance Co., meets the seductive wife of one of his clients, Phyllis Dietrichson, and they have an affair. Phyllis proposes to kill her husband to receive the proceeds of an accident insurance policy and Walter devises a scheme to receive twice the amount based on a double indemnity clause. When Mr. Dietrichson is found dead on a train-track, the police accept the determination of accidental death. However, the insurance analyst and Walter's best friend Barton Keyes does not buy the story and suspects that Phyllis has murdered her husband with the help of another man.