Great-Granny Girdle and Boxer Boy

"“Alright, sonny,” “Prepare to get your bucket whupped by Boxer Boy and Great-Granny Girdle!”"

- Boxer Boy

"“Yes, it is, my big strong man,”"

- Great-Granny-Girdle

Great-Granny Girdle and Boxer Boy are superheroes and grandparents of George and Harold

Captain Underpants and the Preposterous Plight of the Purple Potty People
Good George and Good Harold take the 3D Hypno Ring so that they could dehypnotize Sulu and the Extra-Strength Super Power Juice just in case. However, Mr. Beard stops Good George and Good Harold from leaving the house and forces them to come inside as it is Grandparent's Day and Good George and Good Harold have to eat dinner with their grandparents at Good George's house. While Good George and Good Harold try to explain to Mr. Beard they need to leave, George and Harold's grandparents take the remainder of the Extra-Strength Super Power Juice and drink it.

Elsewhere, Evil George and Evil Harold find Good George and Good Harold's treehouse, rummage through their personal belongings and find the "Goosy-Grow 4000". They then use it to transform Sulu into a giant monster. Sulu charges at Good George and Good Harold But Crackers flies in and Good George and Good Harold flee from their home flying over on Crackers' back to be Chased by Evil Harold, Evil George, and Captain Blunderpants. The Good George and Harold decide that they have to drink the superpower juice to fight back, but unfortunately, they realize it is somehow empty despite there being a third amount of juice earlier. Since there was no more of the Super Power Juice left,

Later Good George and Good Harold are forced to get away on Crackers while Captain Blunderpants flies after them with Evil George and Evil Harold in his hand. Good George soon finds the Shrinky Pig 2000 in the treehouse which the gang can use to make the evil twin counterparts no longer a threat. Unfortunately, it is too late as Captain Blunderpants grabs the duo by their shirts while the device falls into the hands of Evil George and Evil Harold. When all seemed lost at a point before Captain Blunderpants can finish off Good George and Good Harold due to Captain Underpants not being in commission to save them, someone's voice stops him from doing so.

George and Harold's grandparents have arrived and order Captain Blunderpants to put down their grandchildren, but he just tightens his grip on them, so George and Harold's grandparents become Great-Granny Girdle and Boxer Boy. They then place two curtains on them as capes before they defeat Captain Blunderpants and save their grandchildren. Good George notes he figured out what happened to the Super Power Juice that disappeared earlier.

However, Evil George and Evil Harold are prepared to use the Shrinky-Pig 2000 on their alternate twins, but Good Harold claims they can go right ahead as they are holding the device backward. The evil twins stupidly believe this and turn the device around before activating it, resulting in themselves being shrunk. After giving the evil twins the punishment they deserve and George and Harold's grandparents fly off for a romantic dinner,