“Knight and Day” is a playfully off-kilter action caper whose sole purpose often seems to catapult Tom Cruise and Cameron Diaz back into the certifiable A-list of celebrity. The movie is overstuffed with witty banter, breakneck action set pieces and a glossy chemistry between the two stars, yet it all too fleetingly delves into the kind of unhinged insanity its leading man seems to be reaching for.

In a summer awash with disappointing or mediocre films — Pixar’s “Toy Story 3” aside — “Knight and Day” comes as a surprise: a movie as goofy and senseless as its titular pun. Cruise gets this, but he almost gets it too well. Cruise’s wacky supporting turn as movie producer Les Grossman in 2008’s “Tropic Thunder” won him praise left and right for sublimating his movie star glamour for a sickening, foul-mouthed caricature.

If Grossman was everything Tom Cruise isn’t, his lead turn here as Roy Miller is everything Tom Cruise “is” — every smile, every lock of hair, every body motion, every line of dialogue seems to channel “Tom Cruise: Movie Star.” In the movie’s manic opening act, this works wonders and helps give the film its buoyancy, but as it draws farther into the action, Cruise becomes less the film’s center and more just an element of its production.

What keeps “Knight and Day” from being as frenetically enjoyable as it strives to be is that it’s trying to make a major Hollywood blockbuster feel like an improvisation. It wants us to feel continually caught off guard by the eccentricities of the action and the rapidity of the banter, but it can’t help feeling overly familiar.

That’s not to say that the movie isn’t tremendous fun at times. It knowingly plays up its MacGuffin — a central plot element that is both everything the story is about and of absolutely no importance — and sensibly plays the dynamic between Diaz and Cruise’s, which is at times magnetic.

But it’s still two movies at once — a super-serious action spectacle, and a no-holds-barred spy film parody. Take an action sequence set on a freeway late into the film’s first act, in which Diaz’s character drives an SUV down the wrong side of the road as Cruise shoots baddies from the hood of the car.

On one hand, it’s a joy to watch Cruise play straight face (or relaxed and goofy face) to Diaz’s freak-out. On the other, the stunts and chaos are well-staged action, but rarely do these elements seem perfectly conjoined.

Director James Mangold, whose last film was “3:10 to Yuma,” reaches high and manages to control all the elements of the production while still giving his stars space to roam within the frame. Of course, that also ends up being the central contradiction the film poses: Can something this expensive put every cent on the screen in tightly controlled spectacle and still find room for the megastars to comfortably play off each other?

While it may not be a consistent film, it’s still entertaining, and it’s great fun just to watch Tom Cruise play himself with such uncontained glee. The whole thing may feel concocted to its last smirk, but at least it’s a knowing smirk.