I used Star Walk, though I just removed it, but I'll likely add it back soon.
Calisto is about how the Little Bear and Great Bear constellations came to be. Even though it's about mythology and has a lot of humor in it, it's an incredibly sad story, the way it's told in the opera:
Calisto is a young woman and follower of Diana. Zeus comes down one day, sees her, wants a bit of fun, but she refutes his advances, because as a follower of Diana, she must stay virginal. Zeus later transforms himself into the form of Diana, meets Calisto again, and convinces her that it's ok to get it on with her, Diana. Zeus has his fun, goes back to Olympus well pleased, and thinks nothing more of it.
Meanwhile, Calisto, in her post-coital warmth, meets up with the real Diana later, and let's her know that she's up for some more fun, if Diana would like. Diana, disgusted at the suggestion, banishes Calisto from her fold, which in mythology, to be cast out by your god/goddess is a fate worse than death.
Oh, but there's more. Hera, Zeus' wife, finds out about his latest dalliance, zooms down to the woods, finds a forlorn Calisto, calls her bad names for seducing her husband, and turns her into a bear, fated to live the life of a bear until she dies.
Zeus finds out, comes back down, and takes pity on Calisto, and tells her that, regrettably, he can't undo what Hera has done, but that she will be rewarded in the afterlife. He also charges Mercury (I think) to be her constant companion over her bear life, to make sure she is safe and not hunted.
He then tells her her reward: when she dies, she'll be transported to the heavens, to sit by his side forever in the sky, and people will forevermore look up and know that Calisto was his true love.
She became the Great Bear, and the Little Bear is their child, because she got pregnant after being with Zeus.
The way the opera handled the final bit, with her death in the forest, after a long and happy bear life, and her ascension to the heavens was just spectacular. They had projected a giant star map on the roof of the auditorium (which was completely dark), and you really started to feel like you were actually outside after a few minutes, and as she died, the music swelled and swelled and slowly, the constellation of the two bears shone brighter than the rest. I was crying my eyes out!