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 would pass but Harold 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 when they get home, 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. The teachers see both George and Harold and their counterparts which causes them to think they were dreaming and strip down to their underwear, and they get arrested and sent to prison. Mr Krupp, however, is shocked by the presence of two Georges and two Harolds in the same way he is in book 5, and is admitted 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 they realized that they were in danger. They take the Turbo Toilet 2000 one kilometer above the ground and drop him, defeating him. To George's disgust, George and Harold realize that they must have been the result of Sulu and Crackers breeding.
After naming their new pets and putting them to bed, the four boys get ready to face another sequel.