Famed private detective Sherlock Holmes and his friend, Dr. John H. Watson, are visited by Chalmers, an insurance agent, who tells them of the mysterious happenings at Drearcliff House, a seaside mansion in western Scotland. The mansion is the new home of a seven-member private club known as the Good Comrades, and Chalmers explains that one night, after receiving a mysterious envelope containing seven orange seeds during dinner, club member Ralph King was killed in a fiery automobile accident. A few nights later, member Stanley Rayburn received a similar note with six seeds, only to be found drowned ten days later. Chalmer's interest in the matter stems from the fact that the club members have made each other beneficiaries to their substantial life insurance policies. Holmes agrees to take the case when he learns that Dr. Simon Merrivale, a surgeon he once suspected of murder, is a member of the club. Holmes and Watson arrive in Scotland just in time to learn that Guy Davies, a third member of the private club, has been incinerated. Despite the presence of Holmes, Watson and the newly arrived Inspector Lestrade of Scotland Yard, club member Alan Cosgrave's exploded body is found near a shed filled with dynamite. Soon thereafter, club member Capt. John Simpson receives an envelope with three orange seeds, and his mutilated body is later found on the beach, identifiable only by his tattoos. That night, Lestrade receives a note from the local tobacconist, Alex MacGregor, who believes that he has found a clue to the Drearcliff mysteries. When Holmes and the inspector arrive at his shop, however, they find the merchant murdered. Later, Holmes has Watson exhume MacGregor's grave, and they discover his casket is empty. The two men then rush back to Drearcliff House, where they learn that Merrivale has been killed, having been crushed to death by a boulder. The only surviving member of the Good Comrades, Bruce Alastair, is arrested for the six murders. Watson disappears soon after, and Holmes and Lestrade search the mansion for secret passageways, finding one which leads them to an old smuggler's cove. There, they find the abducted Watson, along with Merrivale, Cosgrave, Simpson, Davies, Rayburn and King. Holmes then explains that all of the six had faked their deaths in order to collect the insurance money, having used the corpses of recently deceased villagers in their place. Back in London, Chalmers congratulates Holmes for his work on the case, but the detective insists that the reward money be given to Alastair, as his last-minute warning saved Watson's life.