: On a deserted beach, a person dies whose body is streaked as if someone had whipped him. Sherlock Holmes realizes that the man was killed by a poisonous jellyfish.

The retired great detective Sherlock Holmes lives in a small villa in Sussex, located on a hillside with a view of the English Channel. Not far from his house is the famous school. One morning, on the way to the beach, Holmes meets the school principal, Mr. Stackhurst. Both see the cliff teacher Teacher Fitzroy MacPherson suffering from heart disease. Staggering like a drunk, he falls and before his death manages to say: "lion's mane." The body of the teacher is streaked, as if someone had whipped him. Here comes the mathematics teacher Jan Murdoch, an unsociable, reserved person subject to bouts of rage.

The great detective examines the situation. MacPherson spent a quarter of an hour on the beach. Arriving, he undressed, was about to plunge, but did not have time, he quickly put the Mac on his naked body and began to climb the mountain. There was no one on the beach at that moment. A note from a woman was found in the deceased’s pocket. Mr. Stackhurst reports that there was a close relationship between MacPherson and Miss Maud Bellamy.

Holmes and Mr. Stackhurst visit the girl and see Murdoch leaving her house.When asked by the school principal what he was doing there, Murdoch does not answer, a quarrel erupts between them, and the principal dismisses Murdoch.

Maud Bellamy lives with his father and older brother. She is ready to help the great detective find the killer, despite the discontent of her relatives. The girl says that she and Fitzroy wanted to get married, but hid their engagement because of his uncle. The old man could deprive Fitzroy of his inheritance if he married against his will. Murdoch courted Mod, but after her engagement to Fitzroy, his attitude changed.

A week goes by. The great detective is at a loss, makes inquiries about Murdoch, but receives no results. A police inspector comes to Holmes who suspects Murdoch’s murder. But the great detective has an idea. He recalls the scars on MacPherson's body, as if someone had put a red-hot wire mesh on his back.

The conversation is interrupted by Murdoch with Stackhurst. Murdoch's back is exactly in the same scars. Leaving the wounded man in the care of his housekeeper, Holmes leads the inspector and Stackhurst to the shore and gives the name of the killer: cyanide, or "lion's mane" - the jellyfish brought by the current.

Recalling the last words of MacPherson, Holmes recalls that he had once read about her. Murdoch, who was only an intermediary between Maud and Fitzroy, is fully justified. The great detective admits: because of a dry towel, he decided that Fitzroy did not swim, and did not realize that the danger was not on the shore, but in the water. He often had to make fun of the police, and now cyanide avenged Scotland Yard.