Mordecai Tremaine, a quiet and lonely man, reluctantly went to celebrate Christmas at the luxurious country house of Benedict Graham, because he did not know anyone present except the owner and his secretary. However, he did not have to be bored for long - on Christmas night, a very unexpected and gloomy gift was discovered under the tree - a corpse in a Santa Claus costume. To make matters worse, everything indicated that the killer was still in the house... Mordecai Tremaine, having arrived in a quiet village to visit friends, suddenly finds himself drawn into the investigation of a mysterious murder, the victim of which was the charming Lydia Dare. At first, the police suspect that a famous archaeologist is in love with the victim, with whom Lydia suddenly broke up shortly before her death. But very soon the first murder is followed by a second, and the criminal seems to draw inspiration from a gloomy play staged on an amateur stage by local theatergoers...