Hilliary Clark, gem expert, is robbed of a valuable package of diamonds by his son, Edgar, and Edgar’s crook accomplices. Walter, old Mr. Clark’s favorite son, quarrels with Edgar, and his father takes away his revolver, emptying it of the shells, and letting them lie on his library table. The quarrel has grown out of an insult offered by Edgar to Walter’s wife, Helen, and has no connection with the former’s dual life of crime, which neither his father nor brother suspects. At noon that same day, Walter, entering the library, discovers his father dead in his chair. He opens the safe, finds the diamonds gone, and concludes that Clark has been robbed and murdered. Just then Edgar passes through the hall. Walter fires through the curtains at the supposed burglar. Edgar, unhurt, slips the diamonds into the pocket of his brother’s coat, hanging in the hall, and rushes out for a policeman. Walter is found, the smoking revolver in his hand, the safe door open, his father dead, and the gems in his overcoat pocket. Both brothers are taken to jail. Helen secures a detective who finds, in addition to the exploded cartridge in the gun, a second shell on the floor. He also points out an imprint of the end of this shell on the surface of the mahogany bookcase opposite Clark’s desk. The detective explains how the noon sun, focused on the cartridge, and refracted through the lenses of a pair of spectacles on the desk, has fired the bullet. Walter is released. Later, Edgar’s guilt is uncovered, and he is sentenced to prison.