If there is one vexing problem at the core of American comedy in the 21st century worth talking about now–there’s more than just one, of course–it’s that too many actors, writers, and filmmakers are convinced that audiences will only accept sincere emotion if it’s preceded by an hour of loud and obnoxious behavior that’s meant to be funny. This hounds even the best of modern comedies, those that shoehorn in some attempt at emotional grounding amidst delightfully madcap behavior. But it’s aggressively obvious in the bad comedies or even the mediocre ones. Tammy, the newest Melissa McCarthy vehicle, is blessedly just mediocre, as opposed to the execrable Identity Thief. McCarthy once again plays a swaggering, specifically Midwestern, mistakenly arrogant force of destruction, but Tammy is far less hateful and simply muddled and inconsistent, grasping for a collective audience sadface in between overlong, improv-heavy riffs.

McCarthy is Tammy, a fast-food employee who’s having one of the more cliched examples of a bad day: on the way to work, she hits a deer with her car; after arriving late to the job, she’s unceremoniously fired; her car dies on her on the way home, so she walks the rest of the way back, only to find her husband (Nat Faxon) sharing a romantic dinner with their neighbor (Toni Collette, in a blink-and-you’ll-wonder-why-she’s-here role). Undeterred, she heads to her mother’s house, just a few doors down, promising to leave town and never return. Her mom (Alison Janney) doubts her, but her soused grandma Pearl (Susan Sarandon, with a gray-haired wig so bad it must’ve been intentional) offers Tammy the keys to her Cadillac and a fair bit of cash, as long as they stop at Niagara Falls on the way. This may come as a shock to any viewer not educated in the basics of road-trip movies, of course, but the journey ends up being far more eventful and challenging than either of them might’ve expected.

McCarthy, more than with films like Identity Thief or The Heat, presumably had a serious passion about this story, as she produced Tammy and co-wrote it with her husband, Ben Falcone, who directed it. And yet, for a movie that tries to tell a story that happens to be funny, Tammy is oddly aimless, just about as much as the title character. Pearl’s desire to go to Niagara Falls turns into a desire to simply not be put in an old folks’ home, which turns into a desire to go on a neverending bender, which turns back into a desire to go to Niagara Falls. Tammy herself doesn’t seem to have much drive, and her actions from moment to moment rarely make much sense. Once she and Pearl start their trip, she blows through a stop sign for no reason. Pearl chides her, and then immediately offers her beer or whiskey, which Tammy rejects because she’s driving. But…she just blew through a stop sign. And Pearl just gave her grief for it.

It’s small moments like these that are most baffling, as even the supporting characters’ decisions make little sense, even under the guise of being part of a wacky comedy. Take, for instance, Tammy’s well-meaning love interest (Mark Duplass), who allows his married father to shack up with Pearl, even though…his father’s married to a woman who’s never seen but is described as so sick that she needs to be cared for, at home, every day. Who leaves his sick mom at home to let his dad hook up with someone else? Such weirdly immoral behavior can be funny, but Duplass’s character is supposed to be a wholeheartedly nice guy. This film’s characters aren’t complex; they simply change their motivations on a dime based on the demands of a given scene.

And it’s a shame, because the few moments that are truly enjoyable are those where McCarthy isn’t doing another riff on her character from Bridesmaids. She and Duplass have a couple of sweet scenes, as long as you don’t think too hard about how they ended up sitting inches away from their relatives having sex, or spending time at a lesbian Fourth of July party. McCarthy is talented enough that it’s hard not to feel a bit bad for Tammy when Pearl excoriates her cruelly in front of others, and she’s funny enough that a smattering of her one-liners work. Still, Tammy is a movie that doesn’t know what it wants to be, and hopes that no one will notice that fact; it’s almost like noticing that this is a film where a 67-year old actress plays the mother of a 54-year old woman and the grandmother of a 43-year old woman. The only way to appreciate Tammy, sadly, is to not pay attention to its impact as a whole, and to focus only on the bits that make it a completed work.

Grade: C-