As secret agent Ethan Hunt in "Mission: Impossible III," Tom Cruise gets to show off his "vulnerable" side, which means that for a total of maybe five minutes, his gritted teeth and laser stare are toned down a tad.

Cruise's sleek professionalism as a performer generally carries over into his movies, many of which he produces, and "M:I:III" is no exception. It's an expertly engineered popcorn movie - hold the butter substitute - but it also tries (and fails) to be a love story for the ages.

As the film begins, Ethan has retired from active service in the Impossible Mission Force but, of course, keeps signing on for new missions anyway. He is also set to marry the love of his life, a smiley nurse named Julia (Michelle Monaghan). "Our approach," director and co-writer J.J. Abrams was quoted as saying, "is not to make a movie about a spy, but to tell a story about a man who is a spy."

This man has no trouble keeping a straight face while telling Julia that he works as a traffic manager for the Department of Transportation. (Couldn't he at least have said that he works for Homeland Security?) "M:I:III" resembles "Mr. & Mrs. Smith" except that here the wife is a non-pro (i.e., clueless). When Ethan returns home without a scratch - after a rough night with his buddies blowing things sky high and being chased by rocket-launching attack planes and attempting to extract an implanted bomb from the brain of one of his agents - all he can say to Julia is that he's kind of bushed and that he needs her to trust him. And she falls for it. If this union ever goes on the rocks, I pity the poor marriage counselor.

The effort to turn Ethan into a real live human being instead of a just a lethal acrobat never really succeeds. Probably just as well. After all, who really wants to see Tom Cruise get all vulnerable? As an actor he has two modes: Intense and intenser. This is what makes him a forceful presence in his movies and, ultimately, such an uninteresting one. The "M:I:III" series, minus the heart tugs, is in his most effective range. He reportedly even did many of his own stunts (none of which involve couch jumping).

But the difference between a star machine like Cruise and a great actor like Philip Seymour Hoffman is amply demonstrated by this film. Hoffman is Owen Davian, a bad-boy billionaire who makes most Bond villains look like wusses. Davian is behind a plot to incinerate the world with a substance code-named "rabbit's foot."

Or at least I think that's what he's up to. Hoffman could easily have camped or cartooned up the role but, remarkably, he underplays it. Maybe he was trying to get away from his portrayal of Truman Capote, who, come to think of it, would have made a great Bond villain. Davian is truly scary because Hoffman is truly gifted. He captures the sub-zero psychopathology of a man who gets a kick out of killing.

Besides Cruise, the other actors playing Mission Impossiblers, including Jonathan Rhys Meyers, Maggie Q, and Ving Rhames, are sturdy action figures. Their job is to foil Davian, and at one point, when Ethan's wife is in danger and he plans a daring rescue, Rhames's Luther very sensibly tells him, "There's a point when bold becomes stupid."

That point is never reached in "M:I:III" because, unlike the lovefest between Ethan and Julia, the action scenes were never meant to be taken seriously.

That's why they're fun. When Ethan is swinging over and around the skyscrapers in Shanghai - which the on-screen title helpfully informs us is in China - we're practically in Spider-Man country. If you go to "M:I:III" with no illusions about getting helpful marital tips, you'll have a good time. As Ethan says to his wife, "I need you to trust me on this." Grade: B+

Rated PG-13 for intense sequences of frenetic violence and menace, disturbing images, and some sensuality.

Sex/Nudity: 3 instances of innuendo or implied sex. Violence: 20 intense scenes. Profanity: 25 expressions, including 3 strong. Drugs/Alcohol/Tobacco: 2 instances of drinking.