A quartet of murderous psychopaths break out of a mental hospital during a power blackout and lay siege to their doctor's house.

Jack Palanceas Frank Hawkes
Donald Pleasenceas Leo Bain
Martin Landauas Byron "Preacher" Sutcliff
Dwight Schultzas Dan Potter
Erland van Lidthas Ronald "Fatty" Elster
Deborah Hedwallas Nell Potter
Phillip Clarkas Tom Smith / Skaggs
Brent Jenningsas Ray Curtis