Prewytt Brumblydge, inventor of the Brumblitron, must be found in order to disable the device before it destroys the Earth. This is a job for Mr. Bass, but he has disappeared... so the boys pore over his notebook for clues and go spacefaring to find Brumblydge. This time, instead of journeying to Basidium, they fly to an airless rock named Lepton that orbits 1,000 miles above the Earth's surface. This third Mushroom Planet adventure is illustrated by Louis Darling, illustrator of the Henry and Ramona series.

This novel also introduced the fictional metal Brumblium, a greenish metal, that shows as infragreen on a spectroscope, and is twice the density of uranium.