A London police inspector is patrolling the river looking for smugglers, when he becomes attracted to a woman working as a ship's radio operator. When the woman turns up in possession of some smuggled gems, the inspector must determine whether she knew about them or was just being used. If she is guilty, he must arrest her in spite of his feelings. If she is innocent, he must protect her from the smuggling gang, which is known to silence anyone who might talk.