The Adventure of the Devil's Foot recap

While Holmes is recovering in Cornwall, Mortimer Tregennis reports that his sister Brenda died and his brothers George and Owen went insane during a night at their home. All three showed signs of overwhelming terror. Mortimer is soon found dead in a similar locked room. Holmes connects both incidents to fumes created by burning a rare African poison, a theory he and Watson nearly die testing. Explorer Leon Sterndale then confesses that he killed Mortimer in retaliation. Mortimer had poisoned his siblings during a quarrel over property, murdering Brenda, whom Sterndale loved, and leaving the brothers insane. Sterndale made Mortimer suffer the same fate. Accepting his account and motive, Holmes lets him go.

Holmes and Watson are staying on the Cornish coast so Holmes can recover from exhaustion when the vicar, Mr. Roundhay, and his lodger Mortimer Tregennis ask for help. Mortimer says he spent the previous evening playing cards with his siblings Brenda, George, and Owen. In the morning Brenda was dead and the two men had lost their sanity, all three faces marked by intense fear. The room offers no obvious means of attack. African explorer Leon Sterndale, who had been about to leave the country, returns on hearing of Brenda's death and questions Holmes closely. The case changes when Mortimer is found dead under very similar conditions, alone beside a burning lamp. Holmes collects residue from the lamp and deliberately burns a small amount in his cottage. The fumes almost kill both him and Watson, but prove that a rare African substance known as devil's-foot root caused the terror, madness, and death. Holmes confronts Sterndale, who admits killing Mortimer. Mortimer had poisoned his own siblings during a conflict over family property; Sterndale, who had long loved Brenda in secret, obtained proof and used the same poison to execute him. Sterndale's marriage had kept him and Brenda apart. Holmes accepts the confession and chooses not to turn him over to the police, leaving the retaliatory killing unpunished by law.

Holmes determines that burning a rare African poison called devil's-foot root produced the terror and madness in both rooms. Mortimer Tregennis had thrown the substance into the fire while visiting his siblings, killing Brenda and destroying his brothers' sanity in a dispute over family property. Leon Sterndale, who had secretly loved Brenda for years, learned the truth and forced Mortimer to experience the same death by burning the poison beneath his lamp. Sterndale admits what he did and explains that his existing marriage had prevented him from openly joining Brenda. Holmes, judging that formal proceedings would add little justice, allows him to leave. Holmes and Watson survive their own hazardous test of the poison, and the case closes without an arrest.