When two male students execute a rag against the nearby Calladale women's agricultural college--a prank involving rhubarb and dead rats--the ladies decide to give the men back some of their own. They collect the litter and sneak it over to a pub which happens to be a favorite with the men. Their plans of storing the collection are not successful, however, as the ornamental horse carriage beside the pub where they were going to store the contents is already occupied--with the unrecognizable body of a woman clothed in a Calladale blazer.

​Inquiries at the college reveal that one student, Norah Palliser, has been missing for several days. When Dame Beatrice enters the investigation (at the request of nephew Carey Lestrange, who is teaching pig farming at Calladale) another incident comes to light: days ago, a student returning late to the campus encountered the spectral vision of a cloaked, larger-than-life horseman galloping down the college's moonlit path. Dame Beatrice finds the story most interesting, and other facts soon emerge: Norah Palliser was secretly married to a penniless painter named Coles; she may have been connected with Carey's predecessor, a man with questionable morals nicknamed by the students as "Piggy" Basil; and petty thefts have been occuring within the college.

​The coroner reports death by coniine poisoning, probably extracted from the root of spotted hemlock; there's also the puzzling fact that the victim is physically older than Norah Palliser's twenty-three years. But if the body isn't Norah Palliser-Coles, who is it? And where is Norah? Dame Beatrice travels to Northern Ireland, upper Scotland and southern Italy on her rounds of alibi-breaking, until she is ready to place her cards on the table and reveal the solution.