Daisy Wells and Hazel Wong are spending the Christmas hols in snowy Cambridge. Hazel has high hopes of its beautiful spires, cosy libraries, and inviting tea-rooms – but there is danger lurking in the dark stairwells of ancient Maudlin College. Three nights before Christmas, there is a terrible accident. At least, it appears to be an accident – until the Detective Society look a little closer, and realise a murder has taken place. Faced with several irritating grown-ups and fierce competition from a rival agency, they must use all their cunning and courage to find the killer (in time for Christmas Day, of course).