WARNING: The following contains major spoilers for Batman vs. Teenage Mutant Ninja Turtles.
The Batman vs. Teenage Mutant Ninja Turtles animated movie pits the Dark Knight against New York's sewer-dwelling crimefighters in a crossover for the ages. The film revolves around the Turtles coming to Gotham to track Shredder, who's entered into an alliance with Ra's al Ghul. This causes conflict with the Bat, who thinks they're new villains on the block.
It turns out, Shredder, in exchange for the secrets of the Lazarus Pit, decided to help Ra's combine the mutagen from the TMNT franchise with Joker's Venom so they can seed it throughout the world via cloud dispersion and cause chaos. However, after the Turtles and Gotham's finest put their differences aside and team up to stop the villains, there's a post-credits scene that debuts a brand-new enemy who'll undoubtedly have the deadliest of vendettas against both sets of heroes.
CLOUDY WITH A CHANCE OF MUTAGEN
Batman and the Turtles find themselves at Arkham Asylum going to war with giant, mutated versions of the Caped Crusader's rogues. Joker's been turned into a cobra, Harley Quinn into a hyena and so forth, which leads to a massive brawl. After the "heroes in a half-shell" use a retro-mutagen they cooked up to save Batman himself from permanently transforming into a Man-Bat, along with Damian Wayne (Robin) and Batgirl, they all take down the inmates.
But at this point, it becomes clear Arkham's merely a distraction. When the heroes realize a Cloud Seeder has been stolen from Wayne Enterprises, they track Shredder and Ra's to Ace Chemicals, where the tyrants are enacting their endgame, which will see their clouds mutating the world, starting with Gotham. However, Leonardo ends up defeating Ra's, while Batman and Raphael bring down Shredder in a brutal kung fu battle that pushes them to their limit.
When Donatello and Michelangelo destroy the machine, though, Shredder's knocked away by the explosions and debris falling from the sky into a vat of the newly-concocted acid mix he and Ra's developed. He seemingly drowns before the protagonists' very eyes in what we initially think is a dark ending.
THE SHREDDER WHO LAUGHS
While the Turtles celebrate in the Batcave with Batman and company, eating pizza before they head back to the Big Apple, the post-credits sequence paints a different story as we survey the aftermath at Ace Chemicals. The factory's a warzone and, as we zoom in on the vat Shredder fell into, from the burning rubble, a hand emerges and we see a bigger, badder Shredder from the back arising.
However, as he turns to face the camera, with his mask off, there's a wide, delirious smile on Shredder's face, strongly resembling the Joker. He starts laughing maniacally and the screen cuts to black, revealing the rogue ninja formerly known as Oroku Saki is now a hybrid of the Joker and Shredder. We don't know how fractured the Joker-Shredder's mind is, exactly what the chemicals did to him or which personality will be in control, but after this defeat, the Turtles will have some mopping up to do.
Shredder showed disdain for Joker's antics earlier on in the flick, so it's interesting to see him devolve like this into something he hated. With Ra's in prison, this crazy Shredder might just team up with the Clown Prince of Crime in a sequel, knowing full well it'll enhance his chances of revenge against the Turtles and the Bat, especially as they inadvertently turned him into this monster.
Batman vs. Teenage Mutant Ninja Turtles is now available on Digital HD and arrives on Blu-ray and 4K UHD June 4. The film is voiced by Troy Baker as Batman and the Joker, Darren Criss as Raphael, Kyle Mooney as Michelangelo, Baron Vaughn as Donatello, Eric Bauza as Leonardo and Rachel Bloom as Batgirl.