Holmes is enjoying his retirement in Sussex when one day at the beach, he meets his friend Harold Stackhurst, the headmaster of a nearby preparatory school called The Gables. No sooner have they met then Stackhurst's science master, Fitzroy McPherson, staggers up to them, obviously in agony and wearing only an overcoat and trousers. He collapses, manages to say something about a "lion's mane", and then dies. He is observed to have red welts all over his back, administered by a flexible weapon of some kind, for the marks curve over his shoulder and round his ribs.