Like a lot of others, I think I decided to skip this at the time because I'd (even subconsciously) written M Night Shyamalan off after his continued decline following the high point of The Sixth Sense. But there was enough chatter around it to pique my interest, and when it emerged the film was actually the second instalment in a franchise that included Unbreakable (which I became a big fan of after my second viewing), I knew I couldn't miss it.

It's about Kevin (James McAvoy), a young man with such an extreme case of multiple personality disorder 23 people struggle for supermacy in his fractured mind. To all outward appearances Kevin is doing fine enough in treatment, regularly seeing his psychiatrist (Betty Buckley) in one of his guises as the Brooklyn-esque wise-talking Barry and expressing himself creatively when the mood strikes by painting.

But another of his sides, the cold and officious Dennis, has snatched three teenage girls off the street and is keeping them in what looks like an abandoned factory, locking them in rooms awaiting God-knows-what horrible fate.

While there, Casey (Anya Taylor-Joy), Claire (Hayley Lu Richardson) and Marcia (Jessica Sula) interact with the warm, straight-backed and matriarchal Patricia, the cheeky kid Hedwig, and the no-nonsense Dennis who was apparently in control when Kevin abducted them. Every one of personalities also hint at a malevolent being that's coming closer but hasn't yet emerged, a fearsome thing called The Beast.

The race is on, both for Kevin's doctor to figure him out before he hurts himself or others, and for the three depserate girls to escape before The Beast shows up and exacts whatever terrible treatment the other persoanlities keep threatning in hushed and threatening tones.

The set-up is good, so it's even more disappointing that the movie ultimately isn't very satisfying. The first reason is because the threads don't really add up – if they go somewhere, how or why isn't clear enough.

There are hints throughout the story about how different personalities exhibit or generate different physiologies in Kevin's body when they're in charge, a claim that isn't given nearly enough weight considering it seems to be lynchpin of the whole premise when we see him when The Beast arrives, bulging with muscles and roaring like an animal, climbing effortlessly across a wall like Spider-man.

The fact that it turns out to be in the same universe David Dunn (Bruce Willis) and Elijah Price/Mr Glass (Samuel L Jackon) inhabited in Unbreakable seems to intimate that Kevin is actually a superhero, and that the extent of his multiple personality disorder is some sort of grand advancement of humankind. But that idea isn't really explored (not very clearly that I could see, anyway) or explained and seems to exist only to set up the next instalment.

Another problem is that where Richardson and Sula are everyday teenage girls, Joy as Casey seems to have (though is'a weird complaint to make) too much character. She's the school weirdo, a quiet and reserved girl, only mixed up in the kidnapping because the other girls invited her to a birthday party after they felt sorry for her.

At times she seems to understand and anticipate what Kevin and his various selves will do so much it's almost as if she's in on the whole thing, set to play a much larger role than you realise – perhaps to be revealed on one of Shyamalan's signature twists.

But that all makes it sound far less successful as a story than it is. There's plenty there to sink your teeth into in both story and parable, it just needed to deliver on the promise a bit more.

The single area where it stands out however is performance. It's easy to see why any actor would relish a role like this, and McAvoy is having the time of his life. At its simplest you could say he plays upwards of five roles in the same movie, but it goes much deeper as the various selves inside Kevin react to and affect each other through the prism of what Dennis has done to the girls and what The Beast threatens to do to them.