The Adventure of the Lion's Mane

Sherlock has retired. Now living in Sussex, he's enjoying a pleasant walk along the beach when he runs into a friend, Harold Stackhurst, headmaster at The Gables. The two acquaintances have just started chatting when they're interrupted by one of the teachers at Stackhurst's school, Fitzroy McPherson. The man appears to be in a great deal of pain and only manages to mutter something about a lion's mane before collapsing, dead.
He has strange wounds on his back. Then comes another of the teachers, Ian Murdoch, who claims to have no idea what happened, having just arrived himself. But there is no one else close by. "The Adventure of the Lion's Mane" is part of "The Case-Book of Sherlock Holmes".