Cherish transcends genre to become a uniquely entertaining hybrid,” effuses the official website for Cherish. Here’s the translation from publicity-speak to English: “You’re on your own folks—we don’t know what the hell this thing is, either.”

In a world with so much predigested cinema, you want to embrace such marketing befuddlement as a portent of wondrous things—but different ain’t necessarily better. Every so often, a film will take such a sharp right turn from its initial premise that it threatens to throw the audience out the window. Sometimes, as in the case of Jonathan Demme’s Something Wild or Takashi Miike’s Audition, risk-taking makes for an exhilarating ride. And sometimes it just leaves you dizzy and bleeding by the side of the road, wondering what was so wrong about where you were going in the first place.

As a case study for the latter, witness Cherish, which spends a lot of time headed in a satisfyingly quirky direction. Robin Tunney stars as Zoë Adler, a cubicle-dwelling San Francisco computer animator on a perpetual quest to escape her own anxiety-ridden skin. She immerses herself in the world of sentimental oldies radio and can’t stand to be alone. And she’s not alone one fateful night when, after a few too many drinks, she’s carjacked by an assailant who runs down a police officer and flees the scene, leaving Zoë to take the fall with no evidence of the actual perpetrator. While awaiting trial, she’s sentenced to a fate worse than death: house arrest, with her electronic ankle bracelet regularly serviced by a sad-sack police technician named Daly (Tim Blake Nelson) who becomes one of her few connections to the outside world.

So naturally we’re on our way through a story that will teach Zoë an important lesson about being comfortable with herself. It’s easy to spot Cherish’s central character arc at the outset, but it’s just as easy to get caught up in it. Writer/director Finn Taylor sets Zoë in near-perpetual motion as she plots and plans ever-more-creative ways of expanding her radius of movement. She befriends the wheelchair-bound dwarf in the apartment below (Ricardo Gil), and flirts with Daly as he becomes increasingly smitten with his charge. It’s all very defiantly off-center, but it’s performed with enough gusto that the tics don’t feel like tics. Tunney throws herself into her physical role with such abandon you’re afraid she might hurt something, while Nelson does the deadpan struggle to maintain his dignity to hilarious perfection.

At around the one-hour mark, however, you might realize that Zoë’s aforementioned character arc has already been pretty much completed. She’s a strong, confident woman, with nothing to do but wait for her day in court. The episodic bits involving Zoë’s attempts to get into the open air grow redundant, salvaged only by Tunney’s exuberance. There’s a lot of time left to fill, and Taylor isn’t giving us a really compelling reason for continuing to fill it.

So instead of using that remaining time to develop the charming, awkward romance between Zoë and Daly, Taylor sets Zoë loose on the streets of San Francisco to find the obsessive creep who carjacked her. A character story becomes a race-against-time thriller. Goodbye quirky comedy, hello Silence of the Lambs.

It’s bad enough that the tonal shift could leave your inner ears ringing. It’s worse that, despite Taylor’s every effort to give the film a Run Zoë Run kinetic urgency, the decision deadens nearly everything that follows. For Cherish to really bounce, the carjacker needs to be nothing more than a MacGuffin—the plot device that lands Zoë in confinement. Whodunnit simply doesn’t matter, or at least shouldn’t. Turning Cherish into a mystery proves so fundamentally misguided that you start to wonder whether Taylor only had a great one-hour idea that he felt obliged to pad to feature length.

There’s still a lot that’s right about Cherish. Tunney and Nelson—who could turn into one of our great comic character actors—are too good for too much of the time. But a filmmaker has to know when he’s got a good thing going and resist the urge to do something wild. Finn Taylor takes his sharp right turn and sends a breezy Sunday drive careening into a ditch.