Mr. Bass puts the finishing touches on the boys' spaceship and gives them their instructions:
- Take a mascot.
- Bring back a sample of Basidium air in a canning jar.
- Leave at precisely midnight.
- Return at precisely four o'clock the next morning.
- Keep the written directions for operating the ship with you at all times.
- Never doubt!
Somewhat bewildered, but trusting Mr. Bass, the boys return home and make their final preparations. Each boy brings a huge sack of food. Although David forgets the jar, Chuck brought sugar in a canning jar, so all is well. A hasty trip back to grab Mrs. Pennyfeather, the mascot, and her food, and the boys take off at precisely midnight.
The rest of Part Two covers the trip to the Mushroom Planet, and introduces more characters: Mebe, Oru, and the Great Ta. Mebe and Oru are not-so-wise Wise Men who are in disfavor with their ruler, the Great Ta. Unable to discern why the Mushroom People are dying, the Great Ta has ordered the two Wise Men beheaded.