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".

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.

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.

A note in McPherson's pocket suggests someone named "Maudie" had an appointment to meet him, so Stackhurst and Holmes go to the house of Maud Bellamy, the daughter of a wealthy sailor-turned-businessman.

This, and the memory of the dying man's last words, triggers Holmes to start a new line of inquiry, and he begins to dig through his old books.

After treating Murdoch with brandy, and applying salad oil to the wounds, Holmes leads Stackhurst and Bardle back to the bathing pool.

Holmes shows the group a book by John George Wood,[1] detailing the author's own painful encounter with a Cyanea.

The book's accurate description of the jellyfish stings had put Holmes on the right trail; he had initially failed to consider a sea creature the culprit, because he had mistakenly assumed McPherson never entered the water.

According to Owen Dudley Edwards, the original manuscript of the story indicates that Doyle initially planned to have Holmes chronicle his own defeat.

[9] In an adaptation of "The Adventure of the Lion's Mane" that aired on 21 April 1947, Tom Conway played Holmes with Bruce as Watson.

In the CBS adaptation Elementary, in the introduction of The Geek Interpreter, Sherlock briefly reports solving the 1926 cold case death of Fitzroy McPherson, identifying the Lion's Mane jellyfish as the culprit.