The assistant to scientist Professor Barringer, Louis Crandell lost his right hand in an accident during one of his experiments; in compensation Prof. Barringer designs an advanced prosthetic metal hand for Crandell. The professor tells Louis he will make a more normal appearing hand later, but before that can happen another experiment goes wrong with the result that Crandell finds that whenever he gets a high voltage shock he turns temporarily invisible, with the exception of his metal hand.
Unfortunately the ray also has the effect of driving Crandell insane. He starts stealing, and soon graduates to being a full blown psychotic criminal.
Fortunately however the insanity eventually wears off and Louis makes up for his misdeeds during his madness by turning to fighting crime. Soon the reputation of the Steel Claw brings him to the attention of a British espionage agency known as the Shadow Squad who equip his artificial hand with an array of gadgets.