Chief Inspector Roderick Alleyn is on the hunt for Colombo Dmitri, a well-known London caterer who blackmails his wealthy clients.

Mrs. Halcut-Hackett tells Alleyn about her "friend" who is being blackmailed and has been instructed to leave a purse full of money in a sofa at a concert hall.

Alleyn sends his friend, Lord Robert "Bunchy" Gospell to keep an eye on the purse in order to secure the evidence against Dmitri.

Evelyn, Lady Carrados, throws a debutante ball for her daughter from her first marriage, Bridget O'Brien, with many of London's elite in attendance including painter Agatha Troy, Alleyn's mother and niece, physician Sir Daniel Davidson, Bunchy and the Halcut-Hacketts.

Lady Carrados is also being blackmailed and is instructed to leave a purse full of money in the upstairs sitting room.

Alleyn investigates Donald Potter, Bunchy's nephew and heir who is heavily indebted to a Captain Withers who runs an illegal gambling den.

She did not read the letter, but her stepfather was furious and injured her arm, an act witnessed by Sir Daniel who treated the wound.

Miss Harris said she gave the letter to a man in a car who is actually Sir Herbert Carrados who drove Evelyn to the hospital.

He followed Bunchy out of the party, stunned him with the cigarette case and suffocated him with his cloak which he then mailed to a hospital mission in China.

The book concludes with Alleyn and Troy continuing their tense relationship and sharing an awkward kiss and vague plans for the future.