Spoilers! How that 'Mutant Mayhem' post-credits scene and cameo set up next 'TMNT' sequel

Spoiler alert! The following post discusses important plot points and the ending of “Teenage Mutant Ninja Turtles: Mutant Mayhem,” so beware if you haven’t seen it.

Not only did Teenage Mutant Ninja Turtles fans get a new animated reboot, their infamous archnemesis is also back.

The villainous Shredder shows up at the end of the animated reboot “Teenage Mutant Ninja Turtles: Mutant Mayhem” (in theaters now), the start of a new franchise for those popular pizza-loving karate dudes who busted out in toys, comic books and animation in the 1980s.

"TMNT" was the first thing "Mutant Mayhem" director Jeff Rowe was ever a fan of as a kid, and now he’s one of the guiding forces behind the shelled superheroes’ pop-culture future. "I loved it at 10. Now I love it at a thousand,” he says.

Rowe is part of a creative brain trust – including co-writers/executive producers Seth Rogen and Evan Goldberg – charting the Turtles’ path forward, including a big-screen sequel that Rowe will direct and a Paramount+ animated series.

But before all that, a bunch of important stuff went down in “Mutant Mayhem.” Rowe answers some burning questions about key moments, from a shout-out to a teen movie classic to the last-minute appearance of an old “TMNT” favorite.

What happens at the end of 'Teenage Mutant Ninja Turtles: Mutant Mayhem'?

Isolated in their sewer home by overprotective adopted rat dad Splinter (voiced by Jackie Chan), Mikey (Shamon Brown Jr.), Donnie (Micah Abbey), Raph (Brady Noon) and Leo (Nicolas Cantu) dream of going to high school and being turtle heroes for the people of New York City. The latter happens in the action-packed climax, when the good guys defeat the mutated and monstrous villain Superfly (Ice Cube). That in turn helps them reach their other goal when they take off the masks and excitedly attend classes with human teens.

Going to high school is “the most horrible, degrading thing you can do as a human being. Why would you want this?” Rowe says. “There's just something funny about someone being that removed from society and their only understanding of humanity is through watching high school movies and just thinking that is the coolest thing in the world.”

Why are the Ninja Turtles such big fans of ‘Ferris Bueller’s Day Off’?

Their big wants and needs are revealed early on in “Mutant Mayhem” when the turtles, who are only supposed to go above ground for supply runs, sneak in a viewing of “Ferris Bueller” at a drive-in. (Scenes from the actual movie are shown inset on a big screen in the animated world.) “It provided the turtles with an image that felt absolutely impossible for them to achieve: People cheering for them in the street,” Rowe says.

The filmmakers considered a bunch of other teen movies for the turtle bros to watch, like “The Breakfast Club,” “House Party” and even Rogen and Goldberg’s “Superbad,” Rowe says, “but there's not 15 clean seconds of that film that were usable.”

Another possibility: the infamous scene from “Carrie” where Sissy Spacek’s title character “wins” prom queen and gets drenched in pig’s blood. “Carrie is up on stage and the turtles are like, ‘You earned this, Carrie, you did it!’ And then they go in the sewer before the scene turns. That was hilarious but too meta.”

Review: 'Mutant Mayhem' is the 'Teenage Mutant Ninja Turtles' movie we always dreamed of

Who does MrBeast play in his 'Mutant Mayhem' cameo?

The mega-popular YouTuber (real name: Jimmy Donaldson) voices a character called "Times Square Guy," who interacts with Splinter in an early "TMNT" scene. But "Mutant Mayhem" is full of A-listers, including Rogen and John Cena playing the fan-favorite beastly duo Bebop and Rocksteady, music star Post Malone as the crooning manta Ray Fillet and Paul Rudd as chatty scene stealer Mondo Gecko.

"Thankfully, Seth knows a lot of people and has a lot of great recommendations," Rowe says. "When you sign onto a Seth and Evan film, there's a certain bar of quality that it'll have, and it'll generally be a fun, funny thing to work on."

Is there a post-credits scene in 'Mutant Mayhem'?

Yep! The turtles find their school grooves – Raph’s on the wrestling team, Donnie joins the computer club, Mikey flexes his improv comedy chops and Leo teams with human crush April O’Neil (Ayo Edebiri) to investigate the super-shady Techno Cosmic Research Institute – and all attend the prom where they’re watched by TCRI head honcho Cynthia Utrom (Maya Rudolph). Wanting to capture the heroes in a half shell, she orders, “Bring me the Shredder.” Cut to a mysterious caped and armored figure seen from behind, looking at New York City from afar.

Shredder is the biggest bad of them all in the “TMNT” universe, and Rowe wanted to include him because “he's cool and scary and it felt like a thing to do.” Shredder was actually supposed to be the villain of “Mayhem” before filmmakers pivoted to Superfly and thought “we'll do ‘The Dark Knight’ thing where we save the classic villain for the second film.”

But when the opportunity came up very late in the game to include Shredder in a post-credit tease, Rowe says they “muscled” the final animated shot through production. Rudolph did her line at the last minute, too. “We recorded that one with her in like May or something,” Rowe says. “That was an end-of-the-road final add."

Will there be a ‘Teenage Mutant Ninja Turtles’ sequel?

Rowe wants to explore and begin to unravel the secrets of TCRI in the next film. “A faceless corporation that does not care about human interests and puts its objectives above the greater good felt like a compelling alpha villain and would give us something meaty to dig into in the future,” Rowe says. Plus "Utrom" is an alien species in past “TMNT” incarnations, and Rudolph’s character name “is not an accident. Fans will be able to figure out the connection there.”

The real “magic” of “Mayhem" was getting the four teen voice actors in the same room and writing the story around them, Rowe says, so “the sooner we get to that, the better” for the sequel. That said, “consecutive works of art are most interesting when they take big pivots from what came before and take chances," he adds. "The whole last film was us just trying to excite and surprise ourselves, and we've just got to remember to do that on this (second) one.”

New movies to see this weekend: Watch 'Teenage Mutant Ninja Turtles,' bite into 'Meg 2'

This article originally appeared on USA TODAY: 'Ninja Turtles' spoilers: 'Mutant Mayhem' end, post-credits explained