It's not until the second-to-last shot of the lean, mean, most excellent

The Bourne Ultimatum

that somebody cracks a smile. But still, this clench-jawed thriller - with Matt Damon as the somebody's-messed-with- my-head secret agent known as Jason Bourne - is unstoppable fun.

Jumping from Moscow to Langley, Va., to Turin, to Paris to London's Waterloo Station, all before most flicks have finished their "produced by" credits, The Bourne Ultimatum offers a deftly directed series of bone-crunching fight sequences, high-tech hugger-mugger, and head-spinning chases (on foot, in car, by motorbike). Paul Greengrass, handed the baton by The Bourne Identity's Doug Liman and proving himself up to the job with the second installment, The Bourne Supremacy, is quite simply one of the best action directors working today.

Nominated for an Academy Award last year for his taut, terrifying 9/11 drama United 93, Greengrass combines a documentarian's eye for real-life panoramas (and a doc-maker's agility with hand-held cameras) with a big-screen sensibility and editing chops to die for.

The Bourne Ultimatum is built around three major action sequences - in and around Waterloo Station, on the streets of Tangier, Morocco, and in the glass and steel of midtown Manhattan - and the pacing, the rhythms are nothing short of brilliant. The film doesn't slow down to catch its breath so much as it just shifts into another mode of gallop.

Plot? Tony Gilroy, writer of all three Bourne screenplays, based on the Robert Ludlum best sellers, begins things with our hero on the run in Moscow, breaking into a pharmacy, dressing his own wounds, and experiencing disconcerting, disconnected flashbacks - an interrogation room, a mumbled mantra ("Will you commit to the program?"), torture, torment.

Cut to six weeks later and a British journo (Paddy Considine) meeting with a CIA informant in a cafe in northern Italy. The reporter gleans some prized info - about an agent named Bourne, about an operation code-named Treadstone. The deep-cover antiterrorism boys (and girls) back at CIA headquarters hear about the meeting, and track the Guardian newspaper guy back to Britain. So does Jason Bourne, who wants to know what the journalist knows. Bourne is still asking those big existential questions: Who am I? What am I doing here?

Only, in his case, his memory erased, Bourne truly doesn't know. But shards of history are beginning to surface - those flashbacks, familiar names, a phrase, a voice. Damon, serious and seriously buff, plays the CIA spy with the killer instincts and negative recall with a charismatic brand of anti-charisma. He blends in, like the good spy he's trained to be, but he stands out, like a movie star.

Monitoring from control rooms in Langley and New York is the keyboard-clacking, cell-phone-monitoring, GPS-ready CIA team - led by a terse David Strathairn, and by Joan Allen, whose Pam Landy is torn between a fealty to the feds and a gut feeling that Bourne's a good Joe. "Assets," also known as assassins, are brought in. "Sub rosa collection of bodies" is initiated. Operations, and operatives, are compromised.

And everywhere Bourne goes - Madrid, Manhattan - bullets fly, bodies fall. (One amazing rooftop chase covers row upon row of umber-colored Tangier houses, with the ever-resourceful Bourne plucking items from clothing lines - you never know when a towel will come in handy.) And Julia Stiles, the efficient CIA techie, returns to run tandem with Bourne - her big, blank features like a heroine out of some 1960s Jack Kirby comic book.

The Bourne Ultimatum isn't ultimately about anything. Sure, there's speechifying about a cynical government exerting undue control over its citizens - and where does one draw the line between personal liberty and the public good? - but really this is a film about momentum, gravity, trajectory. It's a physics lesson wrapped up in an espionage thriller, and when director Greengrass yells "Action!" he means it.

The Bourne Ultimatum **** (out of four stars)

Directed by Paul Greengrass. With Matt Damon, Joan Allen, Julia Stiles and David Strathairn. Distributed by Universal Pictures. Running time: 1 hour, 51 mins.

Parent's guide: PG-13 (violence, action, adult themes)

Playing at: area theaters