J R Hollingshead of the Gaiety Theater engaged Gilbert and Sullivan separately (they had not yet worked together) to write a Christmas entertainment for his existing comic theater company.
It was to be in the style of a "Panto," where some men's parts were taken by women, so they could wear tights and show off their pretty legs, which was otherwise frowned upon on the British stage.
Gilbert's libretto was about the Gods on Olympus getting old and tired and wanting to take time off from their duties. When Jupiter runs into Thespis, the manager, and his theater company picnicking on Olympus, he proposes that his company take over the gods roles for a year. They quickly agree at the end of Act I. In Act II, we discover that things have not gone that well, and when the gods return, they are horrified at the company's "experiments." The gods resume their roles and banish Thespis and his performers back to Earth where they are sentenced to become tragedians, whom no one ever goes to see.
Most of Sullivan's music has been lost, except for "Climbing over Rocky Mountain," (which was reused in Pirates) and "Little Maid of Arcadee," (which was published in sheet music form at the time.) Any number of musicians have attempted to reconstuct Thespis, working from Sullivan's hint that he had included music from Thespis in his later works. There are probably twenty existing reconstructions.
We have chosen to use the verson completed by Anthony Baker and Timothy Henty, which was performed by the Savoynet Performing Group at the G&S Festival in 2014. While most of the music is by Sullivan, they did include some of Offenbach's music as well, since we know that the original production quoted Offenbach. Other music is from Pinafore, Pirates, Gondoliers, Princess Ida, Ruddigore, The Zoo, Utopia, Grand Duke, The Rose of Persia, Patience, Cox and Box, The Beauty Stone and a parlor song called "The Chorister." You can see a complete table here.