There is one thing interesting about “Alex Cross,” and if you miss this, you’ve missed the whole movie.

It’s not the story — that’s worse than mediocre. It’s not the lead actor — nothing wrong with Tyler Perry, but as an action star he’s no Vin Diesel. And it’s not the dialogue, which has a clunker every other scene. It’s the direction.

Notice the direction.

‘ALEX CROSS’ Rated: PG-13 Opens today

“Alex Cross” is a good example of what a seriously talented director can do with a heaping pile of garbage. Rob Cohen is the director. He specializes in action and is best known for founding the “XXX” and “Fast and the Furious” franchises. His work is big, aggressive and clear, and he never forgets that if you don’t care about who’s doing the action, you don’t care about the action.

There’s an early scene in which a homicidal lunatic (Matthew Fox) tries his lethal hand at cage fighting. It calls for Cohen to create a world that’s underground and pulsing with energy and forbidden lusts, as if the lid has been pulled off to reveal a churning subculture. It reminded me of the nighttime racing scenes in “The Fast and Furious” and the private club scene in “XXX.”

But Cohen doesn’t get another chance in “Alex Cross” to paint that kind of grand, baroque atmosphere.

By action movie standards, the story is small-scale: One lone demented sadist — a “stimulus seeking, sociopathic narcissistic” paid assassin — is killing powerful people in international finance. And it’s up to Alex (Perry) and his partner, Tommy (Edward Burns) to find and put this guy away.

Cohen lavishes time on the personal lives of the two detectives, because he wants us to know and care about them. Alex has a lovely wife (a radiant and memorable Carmen Ejogo), two kids and a child on the way. And Tommy has a girlfriend, also a cop, whom he’s serious about. As in wedding bells serious.

These men and their healthy, rewarding relationships are effectively contrasted with the personal life of the assassin, well-played by Fox as an insulated, self-protective, hypersensitive study in perversion. Thus, the groundwork is laid. What follows is a succession of atrocities and outrages, while the cops play catch-up and become increasingly frustrated.

Alas the story itself has two big problems. The first is that the audience can barely care about the assassin’s course, because we don’t care about the people he’s murdering.

The second is that when the movie does come up with a way to turn Alex and Tommy’s quest into something personal, it doesn’t do so in a way that energizes the story. There’s no urgency. On the contrary, the story, to its detriment, goes to places that are downright disheartening.

So the director is out there on his own.

Cohen compensates by taking every bit of action and pumping it up. He doesn’t bloat the big moments, but he does get your attention. Even when you know an explosion is coming, for example, the explosion is a little more interesting than you expected. And there’s a sequence on a commuter train that’s meticulously constructed.

In this way a movie that could have been unwatchable almost becomes something to see. At the very least, audiences who stumble into "Alex Cross" will find ways to enjoy it.