The Invisible Monster is a 1950 Republic film serial, in 12 chapters, starring Richard Webb and Aline Towne.

A would-be dictator and scientist, known only as The Phantom Ruler, has developed a formula which, when sprayed on some solid object, renders that object and everything it contains invisible when exposed to rays emitted by a special lamp, also his own invention. Covered from head to toe in formula-treated cloth, he thus moves about unseen, presently with the objective of stealing enough money and formula components to render an entire army of willing followers invisible. Two henchmen assist him, along with several illegal aliens smuggled into the US by him and used to infiltrate, as employees, possible sites for him to later rob while invisible. When he successfully robs a bank vault, an investigator from the bank's insurer teams up with a woman police detective to solve the mystery of the money which to all outside appearances has just vanished. Tracking clues and interrupting other attempts by the Phantom Ruler to commit crimes, the protagonists round up enough evidence that they are not merely dealing with an ordinary crime ring. Eventually they discover the invisibility fluid and lamp, and the Phantom Ruler is killed when he trips over an open high-power electric cable he had laid on the floor of his den to do in the forces of law and order closing in upon him.

Cast

Richard Webb as Lane Carson
Aline Towne as Carol Richards
Lane Bradford as Burton
Stanley Price as The Phantom Ruler
John Crawford as Harrison
George Meeker as Harry Long
text from wikipedia