Everyone in Hit and Run is clearly having a good time. It’s the audience that gets left out of the fun. This hybrid of rom-coms and road pictures is at its best when it focuses on real-life fiancés Dax Shepard and Kristen Bell, who play a crazy-in-love couple living in a small California town mostly enjoying each other’s company. Shepard, who wrote the screenplay, gives their characters — Charlie and Annie — lots of cute things to do and say, and for once the dialogue doesn’t sound cutesy or fake. Their playful pillow talk is genuinely funny and endearing, no doubt based at least in part on the actors’ real-life relationship.

Then Annie gets a job offer in L.A., which forces Charlie to reveal a secret: He’s been living under an alias as part of the Witness Protection Program after testifying on a band of bank robbers he used to run with, and he’s not legally allowed to leave town. Annie is understandably confounded by this revelation. Some “Who are you?” confrontations ensue. But the movie nimbly skips over the hand-wringing, wisely sensing the audience doesn’t really care much about this stuff. The couple decides Annie’s job offer is worth the risk and they go on the run in the 700-horsepower 1967 Lincoln Charlie has kept hidden in their garage all the time they’ve been living together.

These kinds of contrivances would derail a lesser movie (she really never noticed that enormous car before?). But Hit and Run, which was co-directed by Shepard and David Palmer, never takes itself too seriously, so you don’t have to either. As is the norm for the road-movie genre, Charlie’s former cohorts (led by Bradley Cooper) give chase, looking to give the stool pigeon some payback. Tom Arnold, as a bumbling U.S. marshal assigned to keep tabs on Charlie, also joins the pursuit, but he’s such a klutz he keeps losing control of his car and his gun.

The car chases are nothing special — mostly, they consist of Charlie outmaneuvering the bad guys in parking lots and airplane hangars — and there is absolutely no doubt, ever, where the story is going to end up. But Shepard and Bell are immensely likable together: They have a natural ease and rapport that serves the movie well, and you’re happy to watch them work out their differences at 100 mph. Hit and Run isn’t very memorable, but it’s funny and sweet and more engaging than any action-comedy Jennifer Aniston has made in her entire career. For the dog days of August, that’s practically a rave.