A London diamond-house clerk is accused of a major jewellery theft, prompting a private detective to investigate a web of suspicious deaths, disappearances, and false leads. The narrative follows methodical inquiries, disguises, secret allies, and daring pursuits—including a balloon episode and stakeouts—as friends and suspects shift roles. The accused's twin and his devoted companion face public disgrace while detectives set traps and pursue leads. A sequence of near-captures and revelations untangles motive and deception, concluding the investigation and resolving the central mystery.