Val and her friends play a game that allows them to communicate with the dead. The Knock Knock Game turns any doorway into a portal to the hereafter, but the girls soon learn that there are rules that should never be broken.

Through the 1950s and 1960s, and running alongside production of the gothic horror films, Hammer made a series of what were known as "mini-Hitchcocks" mostly scripted by Jimmy Sangster, and directed by Freddie Francis and Seth Holt. These low-budget suspense thrillers, often in black-and-white, typically had a twist at the end of the tale. [Wikipedia]