Captain Harry Lewis, Guy Rolfe of the British East India Company, begins investigating the mass disappearance of over 2,000 locals, a mystery that his superior, Colonel Henderson, would rather ignore in favor of protecting the merchants’ caravans that keep the area running. Henderson, eager to appease the city’s powerful traders, ultimately assigns a man to look into the vanishing caravans, and Lewis expects the job to be his. Instead, the newly arrived, somewhat oblivious Captain Connaught-Smith, Allan Cuthbertson, is handed the assignment, leaving Lewis feeling bitterly disappointed.
Lewis grows convinced that an organized gang is slaughtering both people and animals along the caravan routes, with informants slipping information to their hidden operatives among the merchants. He shares his theories and hard evidence with Connaught-Smith, only to be dismissed as a crank. The tension tightens when Lewis is captured by the Thuggee and sentenced to die by a cobra’s bite; his life is saved by a pet mongoose, a brush with the cult that forces the High Priest of Kali, George Pastell, to release him. Yet Connaught-Smith remains skeptical and openly antagonistic toward Lewis, who ultimately resigns his commission in frustration and decides to pursue the truth on his own terms.
Ram Das, Lewis’s houseboy, believes he has spotted his brother Gopali, who vanished years earlier, and he secures permission to search for him. Lewis soon uncovers a grim thread: Ram Das is killed by the Thugs when his severed hand is thrown through the bungalow window; while investigating for Gopali Das, the Thugs force the new initiate to kill his own brother. In the meantime, the merchants, fearing bandits, decide to unite behind a colossal caravan meant to deter attackers. Captain Connaught-Smith leads this grand procession and, foolishly, allows the stranglers to pose as ordinary travelers and join the party. That night, the Thugs strike with their typical deadly efficiency, and every caravan member, Connaught-Smith included, is slain and buried.
Lewis and Lt. Silver, Paul Stassino, a cult member within the caravan, press on to uncover the truth behind the caravan’s disappearance. Lewis spots the scar that marks Silver as a Thuggee devotee of Kali and shoots him in self-defense. He then discovers the buried corpses and makes a perilous return to the cult’s secret temple, where he is captured and strapped to a burning pyre. Gopali Das, haunted by the death of his brother at his own hands, frees Lewis, who in turn casts the High Priest onto the pyre, and the pair escape amid the temple’s chaos. They race back to the city to meet Henderson, who is dining with Patel Shari, Marne Maitland the merchants’ local representative and a covert informer for the Thuggee cult. Gopali identifies Patel’s chief servant as a Thug, and Patel silences his follower only to reveal his own complicity in the conspiracy.
With the web of treachery exposed, Henderson revokes Lewis’s resignation and elevates him to a new post, recognizing that Lewis’s relentless pursuit helped uncover the threat. The film closes with a reflective note on the broader historical struggle, stating that the Thuggee cult was ultimately wiped out by the British, and closing with Major General William Sleeman’s emblematic line: "If we have done nothing else for India, we have done this one good thing."
