So, what's the big twist?
Like many Shyamalan films, Split is about trauma. The film's teenage protagonist Casey (Anya Taylor-Joy of The Witch) is an awkward misfit and the target of Kevin Wendell Crumb (McAvoy), who suffers from dissociative identity disorder. Kevin kidnaps Casey, locks her and two other girls in a mysterious underground lair, and submits them to his 23 distinct personalities, each of which McAvoy embodies with menacing glee. But Kevin also might have a 24th persona, a terrifying entity known only as "The Beast." Unsurprisingly, as the tension rises, we eventually meet the nightmare.
The Beast is an enormous, roided-up version of McAvoy, capable of crawling up walls, bending steel bars, and feasting on teenage girls with no remorse. As Casey tries to make her daring escape, wielding a shotgun and getting her clothes torn off along the way, she eventually gets cornered by the Beast, who notices some scars from self-inflicted wounds on her exposed skin. According to the Beast's twisted logic, Casey is not "untouched" like the other girls he's killed. She's suffered -- like him -- and is "pure." So he lets her go.
We then see Casey waiting in a cop car as a calming voice says her uncle, who is revealed as an abusive creep in flashbacks throughout the film, is on his way to pick her up. If the movie ended there, this would've been a dark, disturbing final image to a story that mixes real life trauma -- sexual abuse, abandonment, and self-harm -- with familiar horror tropes in an often queasy (and effective) way. That's how the "normal" version of this movie would end. But Shyamalan is up to something far weirder.
After a quick scene where we see McAvoy's villain Kevin talking to himself in a mirror, bragging about his supernatural gifts, we cut to a small diner. As a TV newscaster describes Kevin's crimes, referring to him by a new nickname "The Horde," we overhear whispers about another bizarre bad guy who terrorized Philadelphia 15 years prior. He was a serial killer. He was in a wheelchair. He had a funny name. Suddenly, a man sitting at the diner in a shirt with the name Dunn on it -- and played by a Bruce Willis -- turns to them and says the name of the other villain: they called him Mr. Glass.
Yep, that's the big twist: Split is set in the same cinematic universe as Shyamalan's 2000 super-hero tale Unbreakable. In a move that's perhaps even more ambitious than the title-switch J.J. Abrams played with last year's 10 Cloverfield Lane, the notoriously secret-prone director has slipped a sideways-sequel to one of his most beloved movies into theaters without anyone noticing. It's not just a twist ending; it's a twist beginning too.