One quiet evening, Septimus Culthorpe stumbles into 221B Baker Street. He introduces himself as the leader of the Secret Seven – a mysterious group of seven monks who lead a simple lifestyle and reside in Grimstone Priory, an old cottage in Sussex. Two of the seven monks have been murdered in unusual circumstances. The weapon remains unknown and Culthorpe suspects that both suffered a heart attack. However, he is apprehensive of the secret letter that both men received the day prior to their death. Alarmed by the two horrid and unusual murders in Grimstone Priory, Culthorpe seeks the assistance of the detective and his partner.