Eight people, many of them strangers to one another, are summoned to a ritzy Manhattan penthouse apartment by an unidentified host. Once everyone has arrived, the servants are dismissed and all the doors and windows are automatically locked. The unseen host's voice is then heard emanating from a radio loudspeaker, explaining that all of the guests are old enemies of his, and that all are doomed to die this very evening! A couple of foolhardy souls try to escape, only to be electrocuted by one or another of the mysterious host's booby traps. Who is the "hidden" murderer --- and whoooooo will survive? Remarkably similar to Agatha Christie's Ten Little Indians (which hadn't been written yet!), Columbia's The Ninth Guest was remade -- without acknowledgment -- as the 1939 Boris Karloff vehicle The Man They Could Not Hang.