Three thieves break into the home of old Isaacson, a diamond broker, and rob him of his gems. The butler sees what is taking place and notifies the police. As the result, two of the men are captured after a desperate fight on the housetops. The leader, Snake Henely, escapes by hiding in a water tank. Ruth, the girl detective, and her assistant, Warrington, whose life she had once saved, tackle the job. Under the fire of Ruth's questions, one of the prisoners confesses that Snake has the jewels. Ruth dispatches Warrington and several policemen to Snake's home. Queenie, Snake's sweetheart, foils the police by hiding the gems in a bottle containing milk. The girl detective disguises herself as a woman of the underworld. She cleverly manages to fascinate Snake and at the same time arouses Queenie's jealousy. Her protests falling upon deaf ears, Queenie finally betrays her fickle lover. Thus the police learn that Snake has arranged to dispose of the gems through Machson, a fence. Warrington and a number of policemen descend upon Machson's place, but again their stupidity enables Snake to escape. Once more the girl detective saves the situation. Leading the officers to Snake's hiding place she brings about his capture. But with the recovery of old Isaacson's diamonds, Ruth finds herself confronted by a mystery which promises to be more difficult than any she has hitherto encountered.