The Club of Masks - Allen Upward

A deadly secret lurks behind the glittering facade of London's most fashionable masked club, where aristocrats and artists mingle in a decadent underground world of the 1920s. When a mysterious death occurs during a midnight revel, young Dr. Bertrand Cassilis finds himself drawn into a web of blackmail, hidden identities, and dangerous desires.

Working alongside the brilliant but eccentric Sir Frank Tarleton, Cassilis must untangle a mystery that reaches from the glamorous ballrooms of Chelsea to a crumbling castle in the Welsh borderlands. At the heart of the investigation stands the enigmatic Lady Violet Bredwardine, whose connection to the dead man raises questions Cassilis would rather leave unanswered.

As more victims emerge and dark truths surface about a respected physician's secret life, Cassilis discovers that solving this case may force him to confront his own buried past.