Tom Mix has been called to a large eastern city by his old friend Professor Harvey, to investigate the strange disappearance of radium from various hospitals. Soon after he arrived, over a million dollars worth of radium was stolen from what was believed to be a burglar proof room of a big hospital. While Tom and Inspector Jameson were stood outside the heavy steel door to this room - twenty stories above the ground, and whose only window was barred - the thief managed to somehow get in, steal the radium, and leave a black cat in its place...