The ruins of Fossoway Abbey are supposed to conceal the legendary treasure of the Chelford family, sought by Harry Chelford, last of his line, with fanatical zeal. Due to a hereditary disease, he is liable to fall victim to insanity at any time. His fiancee, Leslie Gine, is more attracted to Dick Alford, Harry's cousin and steward at Chelford Manor. Meanwhile, her brother Arthur, a London lawyer is being blackmailed by his chief clerk Gilder, who wants Leslie for himself. Gilder has purchased some bills of exchange on which Arthur forged the signature of Lord Chelford to pay for his own gambling debts. There is also the 'Black Abbot', a mysterious figure who is seen moving about in the old ruin and supposedly guards the treasure. Inspector Puddler of Scotland Yard and his assistant Horatio are called in when a man is found stabbed to death and they try to solve the mystery. Among the suspects is the butler Fortuna who Puddler recognizes as a former inmate of Dartmoor prison. Mary Wenner, who used to be secretary of Lord Chelford (and an aspirant to become his wife), joins forces with Gilder to find the treasure. They manage to discover some scroll cases containing maps of the abbey but are driven off by the Black Abbot. They later return to find the maps gone. Wenner is shot and killed by Arthur Gine, who himself is later killed in a confrontation with Gilder. Leslie Gine calls off the wedding with Lord Chelford. Lord Chelford, plagued by his deteriorating sanity completely loses his grip on reality when he encounters his mother (whom he believes to be dead) in the grounds of the manor. Lord Chelford shoots his mother and the Black Abbot, takes Leslie as a hostage and hides with her in the tunnels beneath the abbey after also killing Gilder. It turns out there have been two Black Abbots: one was the butler working as an agent for Gilder, the other was Alford, working with the family doctor Loxon to care for the insane Lady Chelford whom they had kept hidden from her