The narrative opens with the escape of John Flack, a notorious criminal mastermind incarcerated in Broadmoor, who nurses a deep vendetta against J.G. Reeder and his betrayer, George Ravini. Meanwhile, Margaret Belman takes up a secretarial post at Larmes Keep in Siltbury, a secluded boarding-house run by the eccentric Mr.
Daver. Despite its tranquil setting, the house hides sinister secrets, underground caves, and a web of dangerous associations linked to Flack’s criminal syndicate. Ravini arrives at the Keep in pursuit of Margaret, unaware of the lethal trap awaiting him.
As Reeder investigates a series of alarming incidents in London and Siltbury, including a daring bullion robbery and attempts on his own life, he uncovers the true nature of Larmes Keep as Flack's clandestine headquarters. The plot thickens with the revelation of Daver's true role and the complex web of identities among the boarders. The tension culminates in a dramatic confrontation at the coastal retreat, where natural disaster and human reckoning collide as the subterranean network collapses, bringing a final resolution to Flack's reign of terror and clearing the path for a new beginning for Reeder and Margaret.
