Secret Agent Bart Fargo chases the kidnappers of the inventor of a death ray.

The elderly O'Hara, owner of a fortune in emeralds, offers half of his estate to whoever manages to get him out of jail. The adventurer Don Casey arrives in the town of Guaynas, ruled by the tyrant Villarde, to fulfill that wish.

Patients and those working in an isolated mental hospitals are murdered, one by one, by a madman lurking in the corridors