Josh Brolin impersonating the young Tommy Lee Jones is worth the price of admission to "Men in Black 3." Dry, drawling, deadpan, he nails the flinty Texan in this sentimental sequel to the sci-fi comedies about secret agents in black suits who save the world from aliens.

We have to meet the young Agent K because an alien serial killer who isn't fond of his nickname, "Boris the Animal" (Jemaine Clement), has traveled back in time to save the arm that Agent K shot off in 1969, and avoid the 40-year prison sentence that followed.

And that bit of time travel means that Agent J (Will Smith) must go back and rescue his future partner, played by Jones through three films as a laconic loner who has saved the world a few times and doesn't brag about it.

Ten years have passed, and everybody involved is much older now. The head of the agency (Rip Torn) is buried in an early scene. Emma Thompson, in the silliest scene of her big-screen career, takes over. But before she can explain to Agent J what Agent K means when he says "Don't ask questions you don't want the answer to," Boris escapes from the super-secret lunar prison, and the Agent K of the past and Earth of the present are in peril.

That leaves an awful lot of this movie in the usually capable hands of Will Smith. But if director Barry Sonnenfeld, who hasn't been funny in this century, has lost his fastball, Smith has lost a step or two running the bases. It's a movie of scattered chuckles before he manages to deliver his first big laugh, 45 minutes in. Sonnenfeld's full-screen close-ups (his theory of comedy is that it is in your face, literally) catch Smith pasting what he has decided is a funny expression on his face, and freezing it through an entire scene.

Still, this is comedy packed with possibilities, many of which pay off. The black Man in Black time-travels to 1969, "not the best time for your people," J is warned. Race isn't his big problem. Racing to save victims of Boris before they get murdered is.

Brolin, as the young Western swing fan Agent K, does that "surly Elvis thing" that Jones mastered for the older K. A scattering of hippies, an impending moon shot and antique future tech (the Men in Black were ahead of their time, even then) are played for laughs.

But there's no urgency to this, no insistence to the humor. Sonnenfeld's whiplash-quick "Addams Family" and "Men in Black" movies were sprints compared to this ambling follow-up.

We find out the secret behind supermodels (they're all aliens) and Andy Warhol (Bill Hader of "Saturday Night Live"). And we meet an alien who sees every version of the future, whose eyes moisten over the coming of the "Miracle Mets" of '69. Michael Stuhlbarg plays this fellow with a Clarence-the-Angel ("It's a Wonderful Life") glimmer and sets the tone for the film's third act, which manages a lump in the throat or two.

Sequels are almost by definition let-downs. But sometimes it's enough to want a movie to work, to want a genuinely funny filmmaker to regain a little of his lost form, for Josh Brolin to come along and firmly take the mantle of man's man/ funnyman from the actor who made humorless so damned hilarious, way back when.