In 1907, some time after his retirement to the Sussex Downs, Holmes goes for a walk and meets his friend Harold Stackhurst, headmaster of a local preparatory school called "The Gables". Shortly thereafter, Stackhurst's science teacher, Fitzroy McPherson, staggers up from the nearby beach, dressed in only his overcoat and trousers. He collapses at their feet, and dies shortly thereafter, having managed to scream the words "The Lion's Mane". He has long, narrow welts curving around his body; he appears to have been repeatedly whipped with some sort of thin, flexible scourge, until his weak heart gave out with the pain.
Ian Murdoch, the morose mathematics teacher at the Gables, arrives on the scene, having just finished teaching an algebra class. Holmes sends him to find the police, then examines the nearby bathing pool, which had been created by unusually high tides after a recent series of gales from the south. Holmes finds McPherson's dry, folded towel, and concludes he had not yet been in the water. There are people far down the beach, and some fishing boats out at sea, but none are close enough to have had anything to do with the tragedy. The police arrive, but a search of the surrounding caves reveals no clues.
