Knight and Day, James Mangold, 110 mins (12A)

We are meant to be dazzled by the sophisticated locations and baffled by the plot, but the real mystery is why anyone bothered

Look up the words "movie star" in your mental dictionary, and there's a fair chance that they'll be illustrated by pictures of Tom Cruise and Cameron Diaz. They may not be the audience-magnets they once were, but there aren't many actors above them on Hollywood's A-list, so when they make a summer blockbuster together, you'd expect it to be ... well, not Knight and Day.

It's not that the film is terrible, exactly. There's nothing in it that prompts you to duck under your seat in embarrassment. Indeed, if it had starred Gerard Butler and Kate Hudson, say, then remaking The Bourne Identity as a limp romantic comedy might have seemed like a reasonable career move. But from megastars of Tom and Cam's wattage, a fully fledged turkey would have been less mystifying than this watchable but unmemorable B-movie.

Cruise plays Roy Miller (the same name as Matt Damon's character in Green Zone, as if the Bourne Identity cribbing wasn't bad enough), and Diaz is June Havens. These two strangers bump into each other in a Kansas airport. Booked on the same Boston-bound flight, once they're airborne, their flirtatious small talk goes so well that Havens excuses herself to check her make-up in the loo. But when she emerges, she finds that Miller has killed the plane's half-dozen other passengers with his bare hands. He is, he explains, a secret agent on the run from a traitor in the ranks. Everyone else on the plane was an assassin, including a stewardess who could, presumably, have saved her colleagues a great deal of bother by poisoning Miller's drink. Anyway, for some reason or other, it's imperative that he and Havens stick together from then on, so off they go on a jet-setting adventure which involves sundry car chases, gun fights, and combinations of the two.

The film's best idea is an over-used but cunning method of hopping between its international locations without losing momentum en route. The trick is that Diaz keeps being sedated, so she can fall asleep in Brooklyn and wake up on a tropical island a second later. Knight and Day also has a sweetly old-fashioned faith in the glamour of European cities. Now that cinema-goers are queuing to see digitally created alien jungles, you have to warm to a film that asks its viewers to swoon over some establishing shots of Seville and Salzburg.

Beyond these two points in its favour, however, Knight and Day has no distinguishing features. It's almost as if the screenwriter jotted down a bare-bones outline of a North by Northwest update, and then, when asked about the specifics, replied, "I'll worry about those later," but never did.

Take the dialogue: none of it is very snappy. And none of the action beats the in-flight scuffle in the opening reel. The MacGuffin, to use Hitchcock's term – the trigger that sets the plot going – is, here, an everlasting battery. But there's no mention of how it works. The scientist who invented the battery, Paul Dano, is a Hall & Oates fan, but that's the closest he gets to a personality. Even the ill-fitting, almost random title sounds like the 11th-hour product of a late-night brainstorming session. And if, as the truism would have it, a thriller is only as good as its villain, then Knight and Day is no good at all. Someone's got a battery that can power a city without ever running out, and the only buyer is a bland Spanish arms dealer (Jordi Molla)? You'd think the bosses of BP could outbid him, for a start.

The director, James Mangold, has a strong track record: he made Walk the Line, 3:10 To Yuma, and Heavy, among other decent films. But in this instance, he seems to feel that because he's offering us the dazzling spectacle of Cruise and Diaz together, everything else is icing on the cake.

He's wrong. There's no doubt that the stars have spent a lot of time in the gym over the years, and Knight and Day has the beachwear shots to prove it. But, ungallant as it might be to say, they've also spent a lot of time in the Californian sunshine, and any film which relies so heavily on their sex appeal should be careful not to photograph their crow's feet in such an unflattering light.

They're both still easy on the eye, of course, but at 48 and 37, they could do with some character traits to supplement their looks, and Knight and Day doesn't provide any for either of them. Does Roy Miller have a weakness for women? Does he prefer his Martinis shaken not stirred? Who knows. All the film does is take the intense, unblinking self-belief that people find creepy in Cruise these days, and then amplify it until it's slightly creepier. His character is a Teflon-coated blank, so sure of his own invulnerability that he doesn't stop grinning even as bullets ping around him.

Diaz would have had more luck generating romantic chemistry with her co-star if she'd been cast opposite the Terminator. As for Cruise, his charisma seemed like an everlasting battery itself, up until recently. Now, though, it's fast running out of juice.

Next Week:

John Walsh discovers whether M Night Shyamalan can break his run of duff films with The Last Airbender, a live-action version of a children's cartoon