London, March 1923. Daisy Dalrymple enjoys Verdi's Requiem in the company of Scotland Yard Chief Inspector Alec Fletcher. The mezzo-soprano role in the opera is performed by Bettina Westley, sister of Daisy's neighbor Miss Muriel. However, the concert ends tragically: at the beginning of the second part, the opera diva falls dead right on stage... The cause of death is poisoning. The poison, apparently, was mixed into the liquor, which she sipped between drinks. But who could dare to commit such a daring crime? Who did the talented, but capricious and selfish girl cross? Daisy Dalrymple again comes to Fletcher's aid. They soon find out that the opera singer had more than enough ill-wishers. And each of them had not only a motive, but also the opportunity to obtain cyanide - the poison that was used to poison Bettina...