Dumb but honest. Like a golden retriever covered in ketchup
Diaz is asked to play one of those uniquely stupid Hollywood-committee takes on a modernized cute-woman-in-peril-who-learns-to-handle-her-own-uzis, a tough-talking Bostonian who raised her younger sister and is totally girly, and cute, and goofy, and likes boys, but wears boots (!!) and fixes up muscle cars (!!). Cruise plays agent Roy Miller, the agent gone rogue who swoops into her life and together, they have a series of absolutely predictable kung-fu & fireball escapades across a series of scenic European rooftops.
Knight and Day attempts to enliven its muddled, characterless story by kicking things off with the zenithal version of one of the tiredest Hollywood tropes - characters "meeting cute" - and it succeeds well enough in its early stages, where Diaz is allowed to stretch her cute-chops talking to herself in an airplane bathroom while Cruise's Miller wreaks havoc outside.
It's all downhill from there, though, as Cruise robotically manufactures a performance that at best is reminiscent of the c*cky, aggressive charm he could exude in decades past. The film trades in its early attempts at characterization for repeated slapstick gags and hard to follow plot twists, all of which it then tries to paper over with breathless action and hokey romance.
To be fair, the film doesn't shoot very high. It wears its low-brow goofiness right there on its sleeve, and it mostly achieves its goal of being a light-hearted, dumb as a rock summer action movie, which is more entirely than can be said for, say, Ashton Kutchers' repellent Killers. See it in a good mood with low expectations and you might be well enough convinced that it's light rather than stupid, fun rather than ridiculous, charming and cute rather than manufactured and plastic.
That last part is probably a stretch though. 4/10