Part of the magic of a superhero movie comes from the idea that even the most ordinary of people are capable of extraordinary things. It’s an alchemy that’s taken for granted — Hollywood’s hunger for comic book movies has found the concept of great power coming with great responsibility being superseded by great power just being great for spectacle — which makes a film like M. Night Shyamalan’s Unbreakable a remarkable anomaly, and makes Glass, a long-awaited sequel, so baffling by comparison.

There’s a thread of tenderness and vulnerability that runs through Unbreakable that is antithetical to the invulnerability suggested by its very title, and by the casting of John “yippee-ki-yay” McClane himself, Bruce Willis, as central hero David Dunn. He’s soft-spoken, uncertain, lonely; his gray existence is the opposite of the colorful caped crusading we’ve come to expect. The classic “superhero moments” that Unbreakable does allow — David’s gradual revelations as to his possible superhuman nature, his first heroic act — are all the more thrilling for how not-super David seems.

At moments, Glass comes close to replicating that sense of sublimity. We pick back up with David, now a known vigilante hero dubbed the Overseer, as he pursues Kevin Wendell Crumb (James McAvoy), who, as introduced in Split, cycles through multiple distinct — and sometimes malicious to the point of cannibalistic — personalities in order to cope with childhood trauma. Nudging the two of them together is the hyper-intelligent and brittle-boned Elijah Price (Samuel L. Jackson), otherwise known as Mr. Glass, the man responsible for the train accident that set David on his present path.

In theory, it’s a natural finale. As characters, David, Kevin, and Elijah hinge on an exaggeration of human nature and the difficulty inherent in finding one’s place in the world, with their alignments putting them on colliding paths. Finding a middle between the more aggressive, out-and-out supernaturalism of Split and the internal, emotional stakes of Unbreakable should bring the Eastrail 177 trilogy to a clean close.

In practice, however, Glass ends up feeling at war with itself. No easily achievable middle ground exists, particularly not when one of the two extremes, Split, is such a knot of thorns already, as it deals (poorly) with dissociative identity disorder, Stockholm syndrome, and the idea that only those who have suffered deserve to live.

Rather than try to parse things out, Shyamalan forgoes compelling idiosyncrasies and attention to detail in favor of broad melodrama and a compulsion to show and tell. Every single curveball in Glass ends up whiffing, as the film takes a few extra beats to laboriously spell out what’s already been implied — each development is shown to the audience, and then explained out loud by one of the characters — halting momentum and deflating any sense of surprise that might have breathed a little life back into the increasingly cumbersome story.

That clunkiness is exacerbated by Glass’ only significant new character. Dr. Ellie Staple (Sarah Paulson) dominates the middle of the film as all three supposedly super individuals are forced to confront the extenuating circumstances of their extraordinary feats and the possibility that they may not, in fact, be special at all.

The idea works on paper. Shyamalan’s heroes are grounded in a tangible reality to a degree that makes their crises of faith particularly potent, but the longer the attempted rehabilitation stretches out, the less interesting and more inert it becomes. No matter how you think it’ll shake out — that they’ll be cured of their superheroic delusions or transcend the boundaries that have been set upon them — that question alone isn’t enough to sustain such a large chunk of a film that has so many other avenues it could explore. The idea of a definitive answer also starts to undermine the “real” trappings of Unbreakable and Split, especially the former’s sense of ambiguity; being able to climb walls or bend steel loses meaning when it’s made the rule rather than the exception.

Despite the film’s density, Glass can’t stand on its own. Worse than that, it throws its own three-film buildup under the bus for the sake of a twist and an open-ended non-resolution that can be seen coming down the pike from miles away. The resulting frustration is all the more vexing for how much of Glass verges upon greatness; Shyamalan has an uncanny sense for how to build up tension, and Glass’s callousness, David’s feats of strength, and the viciousness of The Beast (Kevin’s most brutal and least human personality) are terrifying and thrilling in turn.

That those moments work is thanks to evidence of internal life, which in Glass comes in the form of each “super” person’s “normal” counterpart. David has his son Joseph (Spencer Treat Clark, reprising his role), who is now helping him via earpiece; Kevin has Casey (Anya Taylor-Joy), whom he allowed to escape after discovering her past abuse; and Glass has his mother (Charlayne Woodard), who still loves him despite all that he’s done. The emotional bonds there are the most interesting part of the film, especially as the uncertainty the supposed superheroes and supervillains face begins to bleed out. Joseph, for instance, is forced to ponder whether he’s been deifying his father in the wake of his mother’s death rather than face the reality that David might not be super after all.

Again, it comes down to belief — or delusion — and leaving room for a little mystery. Unbreakable excels in this respect, and the best parts of Glass follow in that film’s footsteps, emphasizing Glass’ yearning, ultimately, not to be alone in being what he perceives to be superhuman, and David’s sense of responsibility not only to the world (or at least to Philadelphia) but to his son as scenes of Unbreakable (and the much younger Willis and Clark) flash before our eyes. They’re normal people searching for evidence of the extraordinary, and it’s near impossible not to hope for them to be bulletproof, to bend steel.

Despite an incredible performance from McAvoy, Split is just as fractious as it is fun, and Kevin and Casey’s inclusion in Glass doesn’t completely mesh, although it’s easy to follow where narrative threads are concerned. For the most part, Unbreakable and Split are like oil and water, but each time Glass seems like a lost cause, Shyamalan pulls something truly affecting — like a wrenching scene in which a young Elijah is practically rattled to pieces at an amusement park — out of his hat. I want to believe in superheroes. It’s just that Glass never quite manages to take flight.