The books feature the debut of George and Harold's new pets Sulu (a hamster with a bionic endoskeleton) and Crackers (a Quetzalcoatlus) who first appeared in the first and second parts respectively.

At demonstration speech day, George and Harold show off the "Squishy"; two ketchup packets under a toilet seat.

Having read the comic, Melvin angrily goes home and builds a super-powered robot to combine himself with using the Combine-O-Tron.

George turns Mr. Krupp into Captain Underpants and the man saves his secretary, Miss Edith Anthrope.

George's suggestion, reversing the batteries in the Combine-o-Tron, surprisingly works, and only three robotic booger globs fly off.

Then he costs the Purple Dragon Sing-A-Long friends their big game when he mistakes their football for a UFO.

Captain Underpants and the boys are cornered at a local store and starts throwing random items stacked outside.

When the boys return to their clubhouse, George finds out that Harold didn't really send Crackers back and had left her with Sulu.

But the next day, when George, Harold and Sulu are ready to take Crackers back where she belongs, the time machine malfunctions.