When Mary, an international art-dealer, arrives home after the unpleasant experience of an aeroplane - on which she hoped to fly to America - catching fire, she has the further shock of finding her house occupied by three "strangers" claiming to be her husband, cousin and sister. Even more mysterious is the fact that they seem aware of details of her family life which could only have been known to her most intimate circle. In fact, they even - apparently - convince a highly suspicious Detective Sergeant.

The nightmare situation becomes more and more complicated, including the sudden appearance of a murdered man, until, with the arrival of Chief Inspector Gale, a series of ingenious twists brings an eventful evening, for all concerned, to a surprising close.