Captain Underpants and the Terrifying Re-Turn of Tippy Tinkletrousers is the 9th book in the Captain Underpants series.
The story continues where the previous book left off, with George and Harold running away from Tippy Tinkletrousers. As they are running away,
the book explains that what happened was something that was not supposed to happen as Tippy and his Robo-pants were not supposed to appear at the point when George and Harold were about to be arrested as they actually came from the future.
The book then explains something called 'The Banana Cream Pie Paradox', which explains that going back to the past and changing something will remain its changed way in the present. Meanwhile, the story switches to what will happen if Tippy Tinkletrousers did not appear, making George, Harold and Mr. Krupp to jail. George and Harold are sent to the Juvenile Hall while Mr. Krupp was sent to the Piqua State Penitentiary.
One autumn afternoon, Tippy (who is also a prisoner from the fourth book) is tasked to build a statue of Warden Gordon Bordon Shmorden who is chief jailer of the prison, Tippy does the task but instead builds "a giant robot suit". On the day Tippy presents his robot suit to Gordon Shmorden, he reveals his work and climbs on to the suit and freezes everyone in his way (the same suit used at the very end of the eighth book). Tippy then grabs Mr. Krupp and forces him to get George and Harold. Tippy soon finds George and Harold and puts Mr. Krupp down, wanting information of Captain Underpants. George and Harold snap their fingers which makes Mr. Krupp turn into Captain Underpants. Captain Underpants gets chased, while Tippy is unsuccessfully trying to freeze him, soon he accidentally freezes his robotic legs and as a last resort, time travels to five years into the past.
The story switches to George and Harold as kindergarteners who have a hard school life due to the fact that Kipper (a school bully and Mr. Krupp's nephew) bullies them along with his friends and that Kipper can easily get out of a bad jam by calling for his uncle. George and Harold find a way to stop Kipper by using an incomplete prank of the ghost of 'Wedgie Magee', which scares Kipper and his gang, so much that they run out of the school.
On Monday morning, Kipper gives a 5 dollar bill to Donny Shoemeyer and promises to return all the money he stole from him. Elsewhere, Kipper's friends are passing out money to the kindergarteners and doing good deeds for them. Kipper and his friends eventually gave back all the money they stole and never picked on anyone ever again. George and Harold would later call off the vengeful wrath of Wedgie Magee after they were certain their enemies were truly reformed. However it is revealed that what happened was what was supposed to happen not what actually did happen. As it turns out, Tippy had time traveled to right when Kipper and his friends were running out of the school.
It then goes back to when Kipper and his friends were running out of the school. At this point, Tippy and his Robo-pants appear. Due to Tippy's dialogue to Kipper and his friends while it was still dark, they mistakenly thought that it's the haunted pants of Wedgie Magee and become insane. Tippy then travels four years into the future. Unbeknownst to Tippy, his actions led to Kipper and his friends being sent to a mental health institute and the police to immediately suspect Mr. Krupp for what happened to the sixth graders (in both the original and changed timeline, Mr. Krupp was unintentionally beaten up by Kipper and his friends while escaping from the school due to a blackout). This eventually led to Mr. Krupp being fired.
Tippy then time travels back to the present time, thinking that kids five years from the past were weird. However, as Mr. Krupp was hypnotized when George and Harold were in the 4th grade, a universe is created where Captain Underpants never existed. Tippy arrives to a present destroyed and overrun by giant evil zombie nerds. After learning of the changes that have happened in the present, Tippy realizes the terrible mistake he made and becomes determined to return time to normal and save Captain Underpants. However, Tippy immediately gets squashed by George and Harold, who are now giant zombies nerds. What remains left of Tippy is a red squishy stain.