Limitless is a pretty good movie. But more than that, it’s a great idea for a movie. Our hero (Bradley Cooper) is bright but lazy – slovenly, even, barely holding his life together. A random run-in with an old acquaintance leads to him testing a miracle drug that makes his brain operate far above its normal capacity by, what else, tapping into his “subconscious” (the old “we only use 10% of our brains myth” in action again. ). He’s instantly hooked. For a few weeks, he lives the sort of life you’d want to with that sort of supermind – writing a novel in only a few days, playing the stock market for patterns, impressing people at parties. But he starts to notice mysterious men tailing him, watching. And then the side effects start to kick in…

The first half to two-thirds of this movie is exactly what it should be. Outside of maybe BBC’s “Sherlock”, Limitless shows us better than any other film the way a brilliant mind works, whipping from zooming shot to zooming shot, with careful sound design for each, making the viewer into the protagonist. We are caught up with Cooper in the trance of this new power, watching each mystery fit neatly together like a puzzle. Then the gloom of the degrading effects of the mysterious drugs takes over, and the movie descends into darkness.

It’s here that things a rough patch. What made the film so watchable at first – the way the film let you see Cooper’s mind at work – has now disappeared as Cooper abandons the medicine and tries to muddle through on his own. We feel his frustration, but the absence of the movie’s gimmick makes it plain that without it, the film is simply a generic, by-the-numbers thriller.

Worse is the rushed, let’s-tie-this-up-quick ending. We jump several months ahead in time, and all of the mysteries of the film are answered – but off screen, during the point we weren’t watching. All that’s left is for Cooper to tell the villain (and by extension, us) what happened in the meantime. It’s hugely unsatisfying, and even if it’s not exactly clear how the movie could’ve gotten around its awkward coda, it’s certainly clear that they should have tried harder. The film does exactly what it had done such a good job of not doing earlier: leaving us outside of our hero’s magnificent brain. A victory’s no fun if you’re not there to be a part of it.