Former UNIT luminary Liz Shaw and her assistant Bayliss are investigating a series of bizarre murders, all committed near a soon-to-be-closed psychiatric hospital. When the hospital is unexpectedly reprieved by rich Industrialist Peter Russell events seem to move out of Liz's control. Are the incumbent director of the clinic, Doctor Dove and his predecessor Doctor O'Kane harboring the killer? What is the centuries-old horror hidden in the grounds? And what exactly is the secret of room zero?

Sunday Night Theatre was a long-running series of televised live television plays screened by BBC Television from early 1950 until 1959. The productions for the first five years or so of the run were re-staged live the following Thursday, partly because of technical limitations in this era, and the theatrical basis of early television drama. Some of the earliest collaborations between Rudolph Cartier and Nigel Neale were produced for this series, including Arrow to the Heart and Nineteen Eighty-Four. The Sunday night drama slot was subsequently renamed The Sunday-Night Play which ran for four seasons between 1960 and 1963. ITV transmitted its own unrelated run of Sunday Night Theatre between 1971 and 1974.