Stowaway to the Mushroom Planet is the second in the series of Mushroom Planet books of Eleanor Cameron, and was published in 1956, two years after the original.
The story opens with Theo Bass, the cousin of Tyco Bass, coming to Pacific Grove, CA and visiting the two boys (Chuck and David) from the first book. He has been a traveller around the world for many years, and when he finds out about the mushroom planet, he decides to rebuild the boys lost spaceship and return to what he knows is his ancesteral home.
At the same time, the boys' letters to a nearby university professor to come and give a lecture to their young astronomer's society. The letter is interrupted by the professor's ambitious young assistant, who comes to Pacific Grove to give the lecture himeself.
The young assistant, Horatio Peabody, ends up going to the Mushroom Planet as a stowaway, and causing quite a bit of trouble there. This book is much more topical than the last one was, as Peabody insists that the Mushroom Planet must be explored and exploited "for the good of science" as well as for his own personal glory. Mr. Peabody ends up commiting an act of sacrilige on the Mushroom Planet that almost gets everyone involved killed, and in general annoys and scares all.
However, by the end of the book, Horatio Peabody learns his lesson about the arrogance of his scientific beliefs, and the situation, overall, returns to equilibrium until the next book.