Stories / The Case-Book of Sherlock Holmes · 1926
The Adventure of the Lion's Mane
In retirement by the Channel, Holmes faces a death by flogging with no assailant — the murderer is Cyanea capillata, a jellyfish, and the clue was in a book.
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⚠️ Spoilers below. The analysis discusses the solution, including who did it and how it's found out.
Summary & analysis
Narrated by a retired Holmes on the Sussex coast: a science master staggers up from the beach, lashed as if by a whip, gasps 'the lion's mane', and dies — with no assailant anywhere on the empty shore.
Holmes weighs and discards the human suspects — a rival in love, a possible witness — because the wounds fit no whip or wire and the shore offered no escape. The dying words and the welts nag at a memory from something he once read.
The killer is Cyanea capillata, the lion's-mane jellyfish, whose trailing stings flayed the swimmer; a second near-victim and a chance recollection of a natural-history book solve it. The murderer is an animal, and the clue lay in Holmes's own reading — knowledge as the final instrument.
Deeper reading
Holmes alone
The Lion\'s Mane is narrated by a retired Holmes on the Sussex coast, without Watson, and solved not by legwork but by memory — the killer is a venomous jellyfish, and the clue was in a half-remembered book. Doyle turns the brain-attic doctrine into a whole plot: the stocked, well-indexed mind outlasts the running legs.
A natural death, not a murder
The satisfaction is that there is no human culprit at all — the terror resolves into biology. It is an unusual, elegiac entry, showing the detective in autumn, his powers now those of recollection and reading rather than pursuit. See our essay on the brain attic for the method it dramatises.
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Techniques on display
Memory & knowledge
The stocked brain-attic, ready for instant recall.
In this story: The solution is recall: welts plus a dying phrase plus a half-remembered passage on Cyanea capillata — the stocked brain-attic yielding the one fact that fits.
Practise: 🏛️ Mycroft's Memory Palace Practise: 🏰 Sherlock's Mind Palace
Elimination
Striking out the impossible until the truth remains.
In this story: Every human suspect is eliminated by the wounds and the empty beach: no whip leaves that pattern and no man escaped unseen, forcing Holmes toward a non-human agent.
Practise what this story preaches: every technique above links to a tool or game built on the same method. The best way to learn the canon is to reason along with it.