The Bourne Legacy (12A)

In a film about cover-ups, the tagline of this fourth Bourne picture – "There Was Never Just One" – could itself be justly accused of concealing a more prosaic truth: "We needed to cook something up once Matt Damon bailed on the franchise." If you've not seen any of the previous adventures of the rogue superagent Jason Bourne, you'll struggle to make sense of The Bourne Legacy. Even if you have seen them it will be a test of your recall, and you'll probably find the law of diminishing returns exerting an irresistible pressure.

Bourne doesn't actually appear in this new instalment, but not because he's dead. In fact, it's the inability of the CIA to kill him off that has created a whole new set of problems for its Dirty Tricks Department. It seems there was a parallel top-secret programme of neuro-experimentation, known as Outcome, which produced a team of superhuman agents à la Bourne; they too have been hunted down and terminated, with one exception. This would be Aaron Cross (Jeremy Renner), whose macho credentials are established within 10 minutes: there he is, swimming bare-chested in the Alaskan wilderness by day and defending himself against wolves by night. He trips across mountain heights as nimbly as a goat, hears incoming drone missiles at a distance of miles, and elaborately self-medicates in subzero temperatures. I bet he laughs in the face of safety razors.

The franchise has a new director as well as star. Tony Gilroy, who had a hand in writing the first three movies, has taken over the hot seat from Paul Greengrass. Not an easy post to inherit, this, given the stunning hyperkinetic style in which Greengrass electrified the action of the second and third Bournes. On the other hand, with Damon's departure Gilroy has been allowed to freshen up the cast. Where Bourne had David Strathairn as a foe, hawklike amid banks of video screens, trilling phones and eager minions of the spy-state, Cross is up against his former controller Byer (Edward Norton), a man so driven and steely he goes running in the rain in the small hours (4.14 am in the film's pedantic timeline).

A flashback to the single encounter between the two finds Byer explaining away an unspecified CIA-backed atrocity to Cross: "We are morally indefensible, and absolutely necessary." It's basically the spooks' charter. Byer it is who has decided that the Outcome programme has been "infected" and needs to be shut down, meaning: all operatives must be liquidated, pronto.

And, because there must be a woman to act as the hero's wingman, Rachel Weisz steps in as Dr. Marta Shearing, a government research scientist who's got tangled up in the conspiracy. She understands the medical intricacies involved in Cross's brainwashing and furthermore knows where to find the "chems" – blue pills, green pills – that will set his brain right again.

Weisz does a neat job as his initially reluctant ally, though by the end she's not required for much more than looking terrified and hanging onto his coat-tails. Renner takes to the central role with assurance, stocky and athletic, less pretty than Damon but more humorous and believable, perhaps on account of a carry-over from his fatalistic bomb-disposal expert in The Hurt Locker. There's just a glint of mania in his eyes, the sort that will enable a man to jump off a building using only the facing walls as his means of descent. And he must be one of the few people on the planet to have force-fed a wolf without getting his arm bitten off.

For all Gilroy's supercompetent handling, however, there remains an air of redundancy about this picture. Back in 2007, when Greengrass made The Bourne Ultimatum, he practically reduced the spy thriller to a near-abstract level of tautness. The whippy changes of location were matched to the fragmented sense of the hero's moral world. Jason Bourne had been tortured, trained and finally traduced by his own masters, who signed off on all manner of murderous wrongdoing in the pursuit of national security. The series credibly traced the way Bourne's confusion and loss of identity turned to rage, and then to retribution.

Here, that dramatic arc has been squashed into a package at once overlong and underexplained. Norton conducts grim-faced backroom confabs with other senior spooks, who loosen their ties and drink heavily as the bad news keeps coming, but it is inert (and sometimes incomprehensible) as drama.

The action setpiece has been the great forte of this series, and Gilroy meets the challenge head-on with a bounding chase over the rickety tin tops of downtown Manila. It's a nod, perhaps even a homage, to the previous film's breathtaking chase in Tangiers, which ended with Damon somehow jumping from a roof to land, with a crash, through a window on the street opposite. This latest is prompted by Cross's glimpsed recognition, from a distance of at least 500 yards, of a would-be assassin. I wondered if this enhanced eyesight were a by-product of his genetic modifications, or if he just had a good memory for nogoodniks.

The subsequent motorcycle pursuit through the streets encapsulates the film as a whole, starting off tense and exciting, losing momentum in its middle section, then skidding to a sudden halt. Huh?

That was also my reaction on David Strathairn and Joan Allen briefly reappearing centre-stage at this movie's denouement. I suppose it is meant to confirm that both plots are happening synchronously, but it feels out of whack, and an awkward note on which to end a long (135 minutes) movie. If the makers have any sense they will close the franchise down – but I wouldn't bet against this Legacy paying out a bit more.