The play opens in act 1, scene 1, with news of a murder in London where the murdered person was heard screaming as the tune "Three Blind Mice" played. Meanwhile, in the country, Mollie and Giles Ralston prepare to greet paying guests at Monkswell Manor. It is snowing, and the expected guests arrive. Then an unexpected visitor, Mr. Paravicini, shows up, saying his car is stranded in a snowdrift and that the snow will soon cut off the manor from the outside world. To make the situation creepier, Mollie and Giles note that all their guests seem unpleasant or strange.
In act 1, scene 2, the police call to say they have found a connection between Monkswell Manor and the murder in London. Sergeant Trotter will ski out to investigate, since the manor is now snowbound. He arrives and provides a backstory that indicates the murder in London has to do with older siblings avenging the death of a younger sibling who died while in foster care. While Trotter is there, the lights go out, and Mrs. Boyle is murdered while someone whistles "Three Blind Mice." The scene blacks out as Mollie sees Mrs. Boyle's strangled corpse in front of the sofa.