Dr. Watson is visiting Sherlock Holmes in Cornwall, where Holmes is convalescing from a recent illness. Watson is surprised to find Holmes deeply interested in a recent local tragedy: the deaths of three people in the Tregennis household. The victims are the father, brother, and sister of Brenda Tregennis, a woman who is engaged to Dr. Leon Sterndale, a famous hunter-explorer. The victims were found dead in their sitting room, with no apparent cause of death. The only clue is a strange footprint in the fireplace, which Holmes identifies as the mark of a rare African poison called "Devil's-foot root."