An obsessed pathologist discovers and captures a parasitic creature that grows when fear grips its host. During its original theatrical run, some of the seats in theaters were installed with William Castle's film gimmick "PERCEPTO," special electric buzzers that were timed to go off during a key sequence in the film when the titular creature escapes into the movie theater in the film... and also the one you're sitting in! Death Certificates with the cause of death listed as "Scared to Death" were also part of the gimmicky prop package, and theaters that were unable or unwilling to install PERCEPTO were required to plant an audience member who would "die of fright" during the film and have to be taken out on a stretcher.