At a glittering London charity ball, a famed diamond necklace “vanishes” from a patron’s throat without a broken clasp, turning good intentions into panic as committees, photographers, and security men perform competence while searching for someone easy to blame; the Belgian Detective and his loyal companion follow the smallest staging details—a planted ribbon, a crest-marked handkerchief, a forged signature, and a courier who collapses after being “helped” too quietly—until the case reveals not a daring ballroom theft but a colder plan of substitution, paperwork leverage, and insurance-ready misdirection, where reputation is the real ransom and a respectable fraudster expects the crowd to chase sparkle instead of truth; The Borrowed Diamonds is an original Golden Age–style mystery about social theatre, scapegoats, and the lethal convenience of a tidy story.