Requiem for a mezzo

In March 1923, the Honourable Daisy Dalrymple takes a break from her writing to attend a performance of Verdi's Requiem at the Albert Hall with Scotland Yard's Detective Chief Inspector Alec Fletcher. The tickets are a gift from Muriel Westlea, Daisy's neighbor and the sister of Bettina Westlea, who will be singing the mezzo role. What should be a pleasant afternoon is quickly disrupted when, during the performance, Bettina falls dead on stage—killed by cyanide poisoning. While it is quickly determined that Bettina's on-stage glass of liqueur was laced with the poison, discovering the person responsible will not prove to be an easy task.
Bettina was neither well liked nor well behaved, and many of her colleagues, associates, and even her family had good reason to want her dead. Daisy, determined to help Detective Chief Inspector Fletcher whether he wants help or not, decides to do some investigating on her own. But with so many suspects, the murderer may well go free....