All About Opera: The Face of Helen & Swan Song by Agatha Christie

It's another special double episode, this one focusing on the world
of opera and Christie's personal love of music. In our first story
we attend Covent Garden with Mr. Satterthwaite and the mysterious
Mr. Quin, while in the second it's a private theatrical
extravaganza starring world-renowned prima donna Paula Nazorkoff.
As anyone familiar with the plots of many an opera will know, it
doesn't take long for things to get rather murder-y....