The Lie Chair (1976)

After their car breaks down on a stormy night, young couple Neil and Carol Croft arrive at the door of elderly Mrs. Rogers and her maid Mildred. They are initially mistaken by Mrs. Rogers for her grandchildren, Robert and Sylvia, who were expected for dinner; when they protest their true identities, each of the women suggests that they are merely putting on an act to comfort the other and ask for the couple's co-operation in this. As the evening progresses, Neil and Carol realise something is very wrong above and beyond the behaviour of their hosts.