Sir James Damery, acting for a very illustrious client, consults Holmes. Violet Merville is engaged to marry the Austrian Baron Gruner, who has murdered at least one wife and yet the foolish, headstrong Violet will not hear a word against him. The client is fearful for Violet's safety should the marriage go ahead. Holmes enlists the assistance of Kitty Winter, a woman whom the Baron used and threw aside, to get access to Gruner's album in which he has catalogued his conquests, to make Violet see him in his true light.

Whilst in Cornwall Holmes is approached by the local vicar to determine how Brenda Tregennis died whilst sitting in a room, playing cards with her two brothers, who were struck down with dementia as a result. The trail leads to an African adventurer and a deadly root known as the Devil's Foot.