Just after learning that Earth's scientists no longer consider him a planet, an unhappy Pluto takes a visitor from Earth on a tour of the solar system, sharing facts along the way.

Infuriated by the declaration that Pluto is no longer a planet, President Penguin and his penguins invade Earth and it is up to Ricky and his robot pal to defeat them--with some help from Ricky's cousin Lucy and her pets.

"Cuando Stink se entera de que Plutón reprobó la Prueba de Planeta por ser demasiado pequeño, no tiene más remedio que ponerse del lado de los planetas chiquitos (y la gente chiquita) de todas partes."--Back cover. When Stink discovers that Pluto has been downgraded from a planet to a dwarf planet, he launches a campaign in his classroom to restore its status to that of a full-fledged member of the solar system.