Equestria had been peaceful for a long while after the Mane 6’s last Unity Quest-- Alphabittle and Queen Haven are eagerly making plans for their wedding, and Allura and Twitch have been slowly accepting their new life in this land… although Allura does think about her brother fondly. But as the Mane 6 begin to wonder how they can find Alterro and get him and his sister back together, the Unity Crystals summon the Mane 6 on a new adventure.

This time our heroes are heading to the city of Piqua, Ohio, where two fourth-graders, best friends and next-door neighbors named George Beard and Harold Hutchins, alongside their winged leopard pal Kendra (who lives right across the street from her best friends), are the top pranksters at Jerome Horwitz Elementary School, and they make comic books about a kind-hearted, yet dim-witted superhero named Captain Underpants, which also helps Kendra escape her discouraging problems at home.

But when their pranks go a bit too far and their mean-spirited principal, Benjamin Krupp, finds the evidence he needs to put the Mane 6’s new friends in separate classes, Kendra, George and Harold hypnotize Mr. Krupp into thinking he is Captain Underpants.

However, even without the mean principal, they discover that having Captain Underpants comes at a steep price, and when the Mane 6, Harold, George and Kendra discover that the evil Professor Poopypants is going to try to eliminate all laughter with the help of school snitch Melvin, and Allura’s long-lost brother Alterro, the Mane 6 will have to help save the world once more, as well as help their new friends see that there may be more to Mr. Krupp than they originally thought.

(S4E1 of G5 Adventures and the season four premiere)