The twelfth installment of the Underpants series, Captain Underpants and the Sensational Saga of Sir Stinks-A-Lot, in which our hero battles the evil, mind-controlling gym teacher Mr. Meaner, comes out on August 25. It’s surprising, simple, and wholly enjoyable. It might not be the best book or the funniest book in the series, but that’s OK, because as it turns out, Stinks-A-Lot is a very important Underpants book. That’s because it’s probably the last one.

I grew up reading about this other caped crusader, from his struggles against Professor Poopypants to his win over Wedgie Woman. His stories now are farther between than they’ve ever been—the series began with eight books in nine years; most recently, just four have come out in that same time span—but returning to the epic of Captain Underpants is like diving into a ball pit. It’s a place where you don’t have to care about anything else. There aren’t any heavy themes or theses, and there’s no pressure to critique or analyze or “put into perspective.” Just toilet jokes. It’s great.

The series has grown increasingly self-aware, most notably of the fact that it’s so sprawling, indulgent, and reliant on past ideas. While this works, Captain Underpants‘s brand of self-deprecation has contributed to another feeling: inevitability. The evil principal Mr. Krupp was always going to hear those fateful finger snaps, and he was always going to transform into his kind-hearted, super-powered, tightie-whities-wearing alter ego. We were always going to hear George yell, “Oh no!” and his buddy Harold finish with, “Here we go again!” as they chase after our titular hero and his triumphant shouts of “Tra-la-laaaa!” As long as the Waistband Warrior could keep laughing at himself (laughing with us, that is), the books were going to keep coming.

But after Stinks-A-Lot’s latest round of pun-laced, winking bathroom humor, the realization hits that maybe this is it. Without spoiling what happens, it’s safe to say that if Dav Pilkey wanted to end his classic kids’ series right here, he could. The ending of Stinks-a-Lot is sentimental, personal, and a little bit emotional too. It reduces the series to what it was about all along: the joy of childhood, the freedom of imagination, and the thrill of adventure. In the wake of all the plot’s wacky time loops and internal references, the book pulls back sharply, and I was struck with why I picked up the original novel in the first place.

Captain Underpants buried a sneaky story underneath all of the slapstick and wordplay. At the series’ core, our two heroes, George and Harold, were just two young boys determined to be kids. They stuck it to all the supervillain adults, they rode their skateboards, they switched the signs around, and they howled and laughed through all dozen adventures. They never matured or evolved or grew up, and while critics might argue that’s bad writing, these books were never for those snobs anyway. These books were fun above everything else, and that was precisely what made them great. Captain Underpants made me laugh and hoot and holler with my friends way back in grade school, and they do the same now. The series was a prime example of reading that made you want to read more.

I wish more books were like these. I’ll miss Captain Underpants and George and Harold a lot. For nearly 20 years (and counting?) they stood as stories that captured the pure joy of reading like few other novels did. Through the ups and downs of the crazy narrative, the heart of Captain Underpants was always the same: be an adult when you have to, but be a child while you can.