Captain Underpants and the Tyrannical Retaliation of the Turbo Toilet 2000, is the 11th epic novel in the Captain Underpants series. It was released on August 26th, 2014.
The story begins where the last book left off with Melvin kidnapping George, Harold, and Captain Underpants. He takes them to the present where Tippy Tinkletrousers is pursuing their past selves to the past. George and Harold ask why Melvin saved them by hacking into the bank's computer and defacing their surveillance photos, because in the past Melvin hated them. Melvin simply replies that 'he had his reasons'.
Meanwhile, the kickball that one of Wedgie Woman's robots kicked into outer space in book 5 was heading towards Uranus and decapitates the Incredible Robo-Plunger. The juice that came from the Robo Plunger poured into the Turbo Toilet 2000's mouth, reviving him. The Turbo Toilet comes back to life and decides to take vengeance on Earth. He dismantles the Robo Plunger, remakes it into a rocket scooter and flies from Uranus to Earth.
On Earth, the police were searching for George, Harold and Mr. Krupp for 'they' robbed a bank in Book 8. However the police gave up because they couldn't find them (they had travelled back in time to the cretacious period) and Melvin was very happy due to the absence of George and Harold. Two weeks later though, the Turbo Toilet 2000 invaded and annoyed Melvin, who had almost completed his project. He insulted the Turbo Toilet 2000, who chased Melvin to Jerome Horwitz Elementary School where he hid in Mr. Krupp's office. He notices a toenail from Mr. Krupp, and he tosses it away in disgust. Suddenly, he remembers that Mr. Krupp is Captain Underpants and he could extract his DNA to him. Melvin started searching for the toenail again. Eventually the Turbo Toilet 2000 finds him, but Melvin finds the toenail, converts it to a juice, and drank it. Melvin then gained Captain Underpants' superpowers and destroyed the Turbo Toilet 2000.
Later on, Melvin becomes a super hero. One year later, he has had enough for the citizens were abusing the system, for requests such as help for dropping their wallet in the toilet. Melvin tracks down Captain Underpants, George and Harold using a GPS device he installed in Sulu's robotic endoskeleton. The story returns to when Melvin has took the heroes back to the present. Melvin warns them that something bad is going to happen in two weeks and teleports to the unknown. When they get home, George and Harold need a lot of sleep due to the fact they had not slept for the past thirty hours. However, their angry parents punish them by making them do chores because they skipped school the previous day in the last book. Unfortunately for them, they forgot the following day was test day so they had to stay up all night and study. The next day, they accidentally oversleep and miss school and the tests, causing the boys to be separated due to the fact George had still passed but Harold had failed.
They decide to use Melvin's time traveling squid suit to go back in time to take the tests. When they arrive in the past, Harold notices they have created an extra Robo-Squid suit (the one from yesterday) but after they go through the school day and return to the treehouse, they discover they have also created another George and Harold (their yesterday selves). They decide to use a system where the Yesterday boys would go to school on even numbered days, and George and Harold would go to school on odd numbered days. The next day, George and Harold are at home, and run out of money. They create a new Super Diaper Baby comic and go to school to sell it. Ms. Anthrope catches them though, so George tries to claim she is dreaming. She doesn't fall for it and demands Ms. Ribble to send George and Harold to her office immediately. This results in her seeing both George and Harold and their counterparts, which she is shocked and confused at. Ms.Anthrope believes that what George said is true and goes running out her office in her underwear. Yesterday George and Harold get mad at the future counterparts for coming to school and George and Harold explain they ran out of food and wanted to sell comic books to make money so they can purchase food. Yesterday George and Harold however take the comics away, so they can sell them and tell George and Harold to get out of the school before somebody sees them.
George and Harold aren't happy at being told what to do and try to cause mischief. However they are seen by the other faculty members and, upon seeing George and Harold in the hallway and their yesterday counterparts in Ms Ribble's room, they instantly believe they are dreaming and proceed in stripping down to their underwear and doing crazy things. Mr Krupp, however, is shocked by the presence of two Georges and two Harolds in the same way he is in book 5. When the police arrive, they arrest the faculty except Mr. Krupp for indecent exposure, reckless endangerment, resisting arrest, and pantsing an officer. The police are unsure of what to do with Mr. Krupp as he didn't actually do anything wrong, but they eventually admit him to the Piqua Valley Home for the Reality-Challenged.
Twelve days later, the Turbo Toilet 2000 attacks the city, and George and Harold make Talking Toilet costumes and lure the Turbo Toilet 2000 to Captain Underpants. The Captain is victorious, but one of the Turbo Toilet 2000's tears lands on Captain Underpants' face and he turns back into Mr. Krupp. Mr. Krupp directs the Turbo Toilet 2000 to the treehouse of George and Harold, where the Yesterday boys reside. The Turbo Toilet 2000 attempts to capture them, but he causes Crackers' eggs to crack and give birth to three strange creatures. At first they take affection to the Yesterday boys, but then they realize that they were in danger. The three hamsterdactyls then attack the Turbo Toilet 2000 and lift him one kilometer above the ground. When the TT 2000 begs to be let go, they do exactly that and drop him, causing him to break apart into many pieces and release Mr. Krupp. The officers assume that Mr. Krupp is another naked teacher that thinks he/she is dreaming, so they arrest him.
To George's disgust, George and Harold realize that the hamsterdactyls must have been the result of Sulu and Crackers breeding. After naming their new pets and putting them to bed, the four boys realize there are too many loose ends in the story. Therefore they must be ready to face another sequel.