One of the plays performed by Master Payne's Circus of Adventure. We haven't seen it staged, but it's important to the plot, because it was a performance of this play that convinced the late, lamented Lars to abandon his life as a cheesemaker's apprentice and join the Circus. Mentioned , as a warmup before Lars and Agatha remember and rehearse some lines from the more consequential Race to the West Pole.
The Girl Genius print novels shuffle things a bit, having Mystery of the Cast Iron Glacier being noticed by Agatha as a new release (at least in Beetleburg's bookstores) as she walks from her home to Transylvania Polygnostic University for what turns out to be the last time. When Lars talks about choosing a career on stage, the play he watched was instead The Heterodyne Boys and Their Anthracite Burning Earth Orbiter, which has never been mentioned anywhere else to date.