The faculty and student body at Sudeley are shocked but scarcely saddened when the headmaster’s obnoxious nephew, Algernon Wyvern-Wemyss, is found dead in a haystack on Sports Day.
But when the young English master, Michael Evans, becomes a suspect in the case, he’s greatly relieved when his clever friend Nigel Strangeways, who is beginning to make a name for himself as a private inquiry agent, shows up to lend a hand to the local constabulary.
Strangeways immediately wins over the students and even becomes an initiate in one of their secret societies, The Black Spot, whose members provide him with some of the information he needs to solve the case.
In the meantime Michael and Hero Vale, the pretty young wife of the headmaster, continue their hopeless love affair. When another murder follows, Strangeways is soon certain of the murderer’s identity, but until he can prove it, he’s reluctant to share his theory with the unimaginative but thorough Superintendent Armstrong.