In Janáček's opera The Makropulos Case, the operagoer must accept a single premise of implausible fantasy: from then on, the action flows inexorably and logically to its conclusion. It's a device used by a great deal of fantasy literature, from the low-brow of The Day of the Triffids to the high-brow of Janáček's compatriot Kafka's Metamorphosis. In The Makropulos Case, the device is deployed with great theatrical power.
SPOILER WARNING: if you don't want to know the plot, don't read on!
The central question of the opera is what would happen if a woman actually found and drank an elixir which made her young and alluring forever (or, in the case of Elina Makropulos, for 300 years). Janáček's answer - protrayed with complete credibility - is that her life and the lives of everyone she touches would be quite dreadful in a whole variety of ways that are thoroughly explored in the opera. All men want to possess her, many want to kill her if they discover they can't. Motherhood ends up as a succession of "thousands of bastards running around the world". If she does find true love, she must inevitably be left to mourn as her lover withers and dies. Even having the same name for that length of time becomes unbearable: by the time we are introduced to Elina, she has become the diva Emilia Marty, having gone through Ellian McGregor, Eugenia Montez and a dozen other aliases (obviously retaining a fondness, however, for the initials "E.M.").
The opera is based on a play by Karel Čapek (also, by the way, one of the early pioneers of science fiction and the creator of the term "robot"). The play is described as a comedy, and there may be plenty of humour around, but the music and the thread of the action tell a different story: if this is humour, it's blacker than black. And Janáček's music is quite unlike anyone else's: without using the atonal harshness of later 20th century composers, it's mercurial and full of invention, continually moving in tone and shape to complement the action.