In his introduction, Holmes informs the reader that he has retired. The man who was so much a part of the bustling metropolis of London has moved away to lead a quiet life on the Sussex coast, keeping bees and going for walks. He tells the reader, “I had given myself up entirely to that soothing life of Nature for which I had so often yearned during the long years spent amid the gloom of London.” This does not sound like the tireless sleuth we have come to know in the previous stories. It is perhaps also surprising to learn that Holmes is now finding comfort in the quiet, seclusion, because in earlier tales, such as “The Adventure of the Copper Beeches”, he expresses a distinct horror of the countryside, fearing the isolation and the feeling that all kinds of criminal activities can take place without anyone finding out.

Watson seems to have slipped out of Holmes’s life: “an occasional week-end visit was the most that I ever saw of him.” His presence is sorely missed, and this tale serves to illustrate the important role Watson plays in the other stories. With Watson as narrator, the reader has a continual witness to the amazing feats of deduction that Holmes pulls off. As Watson is astounded, so too is the reader, creating a sense of both excitement and anticipation regarding what the detective will do next. However, because Holmes regards many of his deductions as commonplace and self-evident, when we see the sleuthing process from his point of view, his discoveries no longer seem quite so marvelous or even surprising. Holmes acknowledges this, saying in his introduction that while Watson would make much of “so wonderful a happening” he, Holmes, has to tell the tale in his “own plain way.” The reader may also find Holmes’s narration less charming than that of the often baffled Watson.

This adventure begins in July 1907. The wind has finally abated after a severe gale and it is a beautiful summer morning. As Holmes takes a morning stroll along the cliff, he meets his friend Harold Stackhurst, headmaster of the local Gables college. Despite being a loner by nature, Holmes perhaps misses his comfortable friendship with Watson, and has found another companion. Holmes tells us that Stackhurst is the only man who “was on such terms with me that we could drop in on each other in the evenings without an invitation,” evoking memories of Watson turning up unannounced at 221B.

Shortly after their meeting, the pair spot a young man they know. Wearing only trousers, a coat, and some unlaced canvas shoes, he staggers up the path and falls down in agony nearby. Holmes and Stackhurst rush to help him, but it is too late. The young man dies, uttering the words “the Lion’s Mane” with his last breath.

As the coat falls from the dead man’s bare shoulders, Holmes and Stackhurst see that his back is covered in long, bleeding lines “as though he had been terribly flogged by a thin wire scourge.” Holmes notes that “The instrument with which this punishment had been inflicted was clearly flexible,” as the markings are curved around the young man’s shoulders and ribs.

The dead man is Fitzroy McPherson, the science master at The Gables college. As Holmes and Stackhurst stand over the body, another familiar figure, Ian Murdoch, the mathematics master from the same establishment, arrives. Holmes describes him as a sinister, aloof, strange character, and tells the reader that Murdoch had once thrown McPherson’s dog through a plate glass window in a fit of temper. Murdoch is clearly a violent man—possibly with a grudge against McPherson. The reader’s interest is piqued.

Holmes looks into the lagoon, puzzling over McPherson’s death. This illustration was included in the first publication of “The Lion’s Mane” in The Strand Magazine.

Murdoch is dispatched to summon the police from nearby Fulworth while Holmes begins his investigation. He sees signs that McPherson had fallen over several times as he ascended the cliff path. Down on the shore, Holmes sees some naked footprints that suggest McPherson had gone into the lagoon in which he was planning to swim. However, his towel is folded and dry, so the detective concludes that he could not have gone into the water. There is no one to be seen nearby and no other clues.

Holmes returns to the body to find the police have arrived. They discover a note in McPherson’s pockets indicating an assignation: “I will be there, you may be sure—Maudie.” When the police search McPherson’s rooms, they find letters revealing a secret affair with local Fulworth beauty Maud Bellamy. It seems unlikely the two would arrange to meet in such a public place as the lagoon if they were trying to keep their affair secret. It then emerges that some Gables students would have gone to swim with McPherson had Murdoch not held them back in class. Holmes pointedly asks if it were “mere chance” that McPherson was alone, throwing the reader’s suspicion on Murdoch. Holmes and Stackhurst walk into Fulworth to talk to Maud Bellamy, but as they approach her house, they see Murdoch emerging. When he rudely refuses to divulge what he is doing there, Stackhurst fires him from the school.

Holmes’s admiring description of Maud is more what we would expect of Watson than the famously indifferent detective, as he notes “her perfect clear-cut face, with all the soft freshness of the downlands in her delicate colouring.” He learns that she and McPherson were engaged, but that they kept it a secret from both of their families to avoid upset. As we learn later, Murdoch has just told her that her fiancé is dead, and she is eager to offer Holmes any help that she can with the investigation, but can offer no real clues. It seems, however, that Murdoch was, and perhaps still is, in love with her. Suspicions are settling on the mathematics master, who, as Holmes notes, has taken the earliest opportunity to get away by provoking Stackhurst into firing him. He demands that Murdoch’s rooms are searched.

Holmes goes home to ponder the mystery, and news then comes that McPherson’s faithful Airedale terrier has been found dead at the lagoon where his master died, its little body contorted in agony. Holmes is at a loss what to think. There is something nagging at the back of his mind, however. He goes for a walk to the lagoon to clear his head, and on his return suddenly remembers what it was: “Like a flash, I remembered the thing I had so eagerly and vainly grasped.” He reminds the reader how his brain is like a “crowded box-room” packed full of data, or “out-of-the-way knowledge,” that might one day come in useful. This idea of the “box-room” (or “brain attic” as Holmes also describes it) dates back to the very first Holmes book (A Study in Scarlet), and is a key image Conan Doyle uses to describe Holmes’s way of thinking, which various psychologists have embraced. Russian-American psychologist Maria Konnikova, for instance, has adopted the “brain attic” metaphor as a useful way of understanding how humans store information, organize knowledge, and use it to devise strategies for clearer thinking and “mindfulness.”

In this particular case, however, the reference to the “brain attic” merely serves to remind Holmes to look in his real attic, in which he finds a little “chocolate and silver” book, entitled Out of Doors. Written in 1874, this was in fact a genuine publication by a popular Victorian natural history writer at the time, John George Wood (1827–1889).

John George Wood, the author of Out of Doors, wrote several books. He was a parson-naturalist: a clergyman who viewed the study of natural science as part of his religious vocation.

Having verified his suspicions by consulting his book, Holmes has, to all intents and purposes, solved the mystery of McPherson’s death. But Conan Doyle sustains the story a little longer, tantalizing the reader by introducing a sequence of obstacles that prevent Holmes from revealing his great solution.

First comes Inspector Bardle, who Holmes describes as “a steady, solid, bovine man,” to suggest that he is trustworthy but not especially intelligent. He asks for Holmes’s opinion as to whether he should arrest Murdoch before the suspect leaves town. The fact that Murdoch is hot-tempered, has argued with McPherson in the past, is in love with Maud, and is preparing to leave Fulworth—combined with a lack of any other likely suspect—all indicate guilt to the inspector. But Holmes points out that Murdoch has an alibi, and that the evidence against him is flimsy. The great detective also tells him that he has examined a photograph of McPherson’s wounds, and teases the inspector—and the reader—with possible explanations for their strange nature. Then, just as he is about to explain the truth of the matter to him, Murdoch bursts in and delays the revelation further.

Murdoch is in a bad way. He is in terrible agony, branded with the same weals on his shoulder as McPherson. After knocking back several large doses of brandy, he finally falls into an unconscious stupor. Stackhurst, who had met Murdoch on the cliff and followed him in to Holmes’s house, pleads with Holmes to save them from this apparent curse.

Finally, Holmes relents. “We will see if we cannot deliver this murderer into your hands,” he announces, leading the inspector and Stackhurst down to the lagoon on the shore. As Holmes scans the pool below, he yells triumphantly, “Cyanea! Behold the Lion’s Mane!” and in the water all three men spot the tentacles and globular body of a giant jellyfish. Holmes spies a large boulder above the pool and, at his call to “end the murderer forever,” between the three of them they roll the boulder into the pool to kill the jellyfish.

It seems there was no murder at all and that McPherson, his dog, and Murdoch were all victims of a natural hazard—the sting of the jellyfish Cyanea capillata, commonly known as the lion’s mane. As Holmes shows his friends in his book Out of Doors when they get home, McPherson is not the first to encounter this dangerous giant jellyfish: author John George Woods explains how he had once had a close encounter with a lion’s mane and was lucky to escape with his life. His book warns swimmers who see a tawny-colored membranous mass that resembles a lion’s mane, “Let him beware, for this is the fearful stinger, Cyanea capillata.”

The stings of these jellyfish are extremely painful, but rarely fatal. However, Conan Doyle is careful to stress that McPherson had a weak heart, and so it is entirely plausible that the stings could kill him, while Murdoch survives the attack.

In this story, Conan Doyle has employed a fascinating, real-life killer, and Inspector Bardle is full of admiration for the way Holmes has gotten to the bottom of the case. “I had read of you, but I never believed it. It’s wonderful!” the inspector cries. But as Holmes modestly admits, “I was slow at the outset—culpably slow,” and certainly this is a tale that has not shown him at his most perceptive.

Holmes assumed from the start that McPherson’s death was murder and that there was a human killer, possessing a flaying instrument, to track down. At first he suspected Murdoch of being the murderer, but this was based largely on the man’s appearance and character rather than fact—the type of red herring that Watson might have fallen for.

Holmes was misled, he says, by McPherson’s dry towel: it made him think that the dead man had never been in the lagoon, and claims that had he found him in the water the true cause would have been clear. However, the observant reader will note that Holmes must have failed to spot that when McPherson died his hair was surely still wet, and that his clothes would certainly have been damp had he thrown them on without drying himself first. Also, when Holmes inspected the crime scene he somehow missed the huge jellyfish. But in spite of these slips, it is ultimately only Holmes who solves the puzzle.

Without Watson as Holmes’s foil, Conan Doyle may have forfeited a little of the detective’s brilliance to the plot. But any fears the reader may have that Holmes’s powers are waning with age and retirement can be allayed: his dramatic undercover work in “His Last Bow”, set seven years after “The Lion’s Mane,” shows Holmes at the height of his powers.

Holmes’s Sussex home, like these coast guard’s cottages, looks across the channel, taking in a view of the chalk cliffs. One of Holmes’s pleasures is walking the cliff path to the beach.