John Brandon, an American millionaire, has been married seven times but never found love. Then, when he is in Paris, Mona de Briac comes into his life. Mona comes from an noble family who is facing ruin.

Dr. Janos Rukh discovers a certain type of radium that has almost magical healing properties. But the element has a dangerous side, too, and it has already started affecting Rukh. Consumed by paranoia, he begins to suspect that his wife is having an affair. Wild for revenge, Rukh hatches a deadly plot...using his own poisoned body as a weapon to kill.

A woman's love for and devotion to a married man results in her being relegated to the "back streets" of his life.