In “Knight and Day,’’ it’s mostly for the better. The movie’s a piece of high-octane summer piffle: stylish, funny, brainless without being too obnoxious about it, and Cruise is its manic animating principle. Whenever he zigs or zags, the film races to keep up with him, jettisoning cars, helicopters, and human beings as necessary. At the same time, “Knight’’ tweaks the actor’s “Mission Impossible’’ seriousness by giving us Roy Miller, a super-spy so capable it makes him giddy with delight. Roy’s high on himself, and for the first time in a Tom Cruise movie, we’re meant to laugh. “I’m pretty good at what I do, June,’’ he tells the movie’s heroine, and the gag is that for once Roy’s being modest.
Cameron Diaz plays June Havens, a nice, no-nonsense Boston girl — she restores vintage cars for a living and from what I can tell lives in East Boston — who meets Roy on a homebound flight that quickly turns into a neatly choreographed action-movie parody, the agent dispatching a plane full of baddies while June’s in the bathroom, hyperventilating after flirting with him across the aisle. The sequence sets the stakes for “Knight and Day’’ — things happen much too quickly for us normals to take in, there will be casualties, we’re not meant to take them seriously, and whatever happens, Tom’s on top of it.
Roy is a federal agent protecting the film’s twin MacGuffins — a perpetual energy source no bigger than a battery and the doofus young scientist who invented it (Paul Dano with weedy facial hair) — from a team of rogue agents led by Fitzgerald (Peter Sarsgaard, who doesn’t look at all happy to be here). June is collateral baggage forced to come along for the ride after she and Roy meet cute while landing that plane in a cornfield. Believability is not this movie’s strong suit, but fledgling screenwriter Patrick O’Neill and director James Mangold (“Walk the Line,’’ “3:10 to Yuma’’) work like crazy to keep you from caring.