Grade 5–8—As the Mumby family gathers to celebrate Christmas at Larklight, their Earth-orbiting home, a dire message arrives alerting them of a "Great Danger" threatening the universe. The family sets off across the solar system to Georgium Sidus (aka Uranus) to find out what is wrong. So begins Art and Myrtle Mumby`s latest space adventure. The danger turns out to be the Mothmaker, a malevolent Shaper or creator of planetary systems. It is determined to conquer the worlds, including Earth, created by another Shaper, Mrs. Mumby, and enslave the creatures, including humans, created by her. A series of exciting and entertaining episodes involving various characters and imaginative creatures leads up to a final confrontation between the Mothmaker and Art, Myrtle, and their companions at Balmoral Castle. While the story is told primarily from the point of view of Art, who is prone to use inflated language, Myrtle`s diary entries are inserted throughout and include amusing commentary.