As refreshing as a cool drink of lemonade on a hot day, “Knight and Day” has just what we want from a big summer Hollywood summer movie: glamorous locations, even more glamorous movie stars, lots of crazy stunts and chases, a couple of romantic smooches, and of course some really big explosions.

It helps to have Tom Cruise and Cameron Diaz, two stars with smiles that can fill a 40 foot screen. Wisely, screenwriter Patrick O’Neill and director James Mangold (“Walk the Line”) create the whole movie around what these two performers do best, which is have fun on screen and make it fun for us, too. Their effortless charm gives off enough sizzle to keep things going between stunts.

Cruise plays Roy Miller. We see right away that he’s really good at hand-to-hand combat and running and jumping and shooting. And apparently he is also very good at lying, which is why we do not know whether to believe him when he says he is the good guy. Neither does the person he says it to, a vintage car restoration expert named June (Cameron Diaz). They first run into each other, literally, at the airport. She is on her way to her sister’s wedding. Coincidentally, they bump into each other twice, once before going through TSA security, once on the other side. Don’t think it’s a coincidence? You just might be right!

Cruise and Diaz seem a bit relieved to have a chance to be silly in a summer popcorn movie, after less-than-successful efforts at serious drama (“Valkyrie” and “My Sister’s Keeper”). Both are adept at hitting the sweet spot that keeps the tone light, even in the midst of mowing down bad guys. There’s a lot that doesn’t work in this movie, including repeated scenes of characters being drugged (though Diaz makes a truth serum scene amusing with her matter-of-fact delivery), a distracting amount of jet-setting all over the globe, and literal overkill of legions of bad guys. That title, really? And surely they could do better for the talented Paul Dano (“There Will be Blood” and “Little Miss Sunshine”) than yet another nerdy genius with a universally coveted invention. But there is a lot that works. Diaz and Cruise have a natural chemistry and easy equality that is just plain entertaining. They bring enough conviction to the story to keep us believing and rooting for them without taking it so seriously that we start second-guessing the storyline. Supporting players include the always-welcome Viola Davis, Peter Sarsgaard, Celia Weston, and Marc Blucas. The stunts are expertly staged with Cruise and Diaz clearly right in the middle of it all, not checking their email back in the trailer while stuntmen do anything more strenuous than opening a door, and I have to say, they are summer movie crazynutsfun. It’s not as easy as it looks to combine action, comedy, and romance (I’m talking to you, Killers). But Diaz and Cruise show us how both characters want more of what the other has, without making too big a deal out of it. And they make it seem not just normal that you’d interrupt a deadly chase to capture a hugely valuable little doo-hicky to attend a wedding, but rather sweet.