Debbie Reynolds is nowhere to be found but there is a convenient bachelor in “Tammy,” in which Melissa McCarthy does a better job than she’s previously managed of presenting the screen persona she’s obviously aiming at—a feminine version of Jackie Gleason’s Ralph Kramden, the heavy-set, congenitally frustrated, blustering person who in a pinch turns out to have a sweet, sentimental side. The picture, written by McCarthy with her husband Ben Falcone (who also makes his directorial debut), is also a road movie in which she teams up with Susan Sarandon, whose presence immediately evoke memories of “Thelma and Louise” to mind (in fact, this might have been titled “Tammy and Pearl”). The result is too scattershot and episodic to work as a whole, but overall it’s a better vehicle for McCarthy than those she’s shouldered before.

McCarthy plays the title character, who’s introduced, in a series of scenes designed to appeal to her core audience, as having an unremittingly awful day. Driving to her job at fast-food restaurant Topper Jack’s in her beat-up Corolla, she smashes into a deer that she manages to resuscitate at some loss to herself. That includes her car and her job, where her obnoxious boss Keith (Falcone) cans her for tardiness, leading to the first of several hissy-fit sequences in which McCarthy trashes the joint with her typical abrasiveness. Returning to her little box house unexpectedly, she finds hubby Greg (Nat Faxon) sharing a romantic meal with next-door neighbor Missi (Toni Collette), which sends her into another frenzy. She stalks down the road to her mother Deb (Allison Janney), and when Deb refuses to hand over her car keys, agrees to accept her cantankerous grandmother Pearl (Sarandon) as a passenger on her flight to freedom when the old lady not only offers the use of her vehicle but flourishes a wad of more than six grand cash as an inducement.

This set-up, or prologue if you prefer, does not bode well for the rest of the movie. It seems designed simply to give McCarthy the chance to do a lot of the huffing and puffing that’s disfigured much of her past work, and though there are moments when she’s allowed to show a more vulnerable side, they’re few indeed. Things don’t improve in the next episode, in which, supposedly as a result of going off course in the trip they’ve to take to Niagara Falls, they wind up in Missouri (apparently to permit a gag about Mark Twain at Tammy’s expense—but the itinerary is never made very clear), and McCarthy jumps on a jet ski for a destructive slapstick ride that costs Pearl most of her bankroll. Up to this point the movie has just been more of the same McCarthy who proved irritating in the past.

Things improve somewhat from this point, however, as the bickering duo make their way, apparently to Kentucky, where boozy, horny Pearl takes up with womanizing farmer Earl (Gary Cole), leaving Tammy to be consoled by his nice mope of a son, Bobby (Mark Duplass). There follows further sequence centered on Pearl’s alcoholism that lands both grandma and Tammy in the clink—and when Pearl bails her granddaughter out, Tammy tries to secure the funds to do likewise by robbing the local Topper Jack’s franchise wearing a paper bag ion her head—a scene, featured in the trailer, that seems to go on interminably and lands precious few laughs, sacrificing much of the goodwill the picture had recently won.

Happily, matters get better again with the arrival of Kathy Bates as Lenore, a rich cousin of Pearl’s who takes the pair to the handsome estate she shares with her significant other Susanne (Sandra Oh). There they join in a big July 4 lesbian shindig where both voice their grievances against one another and bond before Tammy’s carted off for robbery and emerges from jail to the arms of her understanding dad (Dan Aykroyd) ready to reconcile with her mother and grandmother while breaking things off entirely with Greg. Of course, sweet Bobby’s waiting in the wings.

The problem with “Tammy” is that it suffers from the need to give audiences plenty of the brash, bellicose McCarthy they’ve come to expect while providing her with a more consistently likable, sympathetic side. The material focusing on the latter is obvious and heavy-handed, but it does serve to mitigate the boorishness of that concentrating on the former. It’s a mixture that’s hard to balance right—and “Tammy” doesn’t manage it particularly well, in contrast to “The Honeymooners,” for example, did in the fifties. (The horrible movie based on Gleason’s program was entirely another matter.)

Still, the movie represents an advance, even if only a partial one, for McCarthy, who’s more engaging here than she’s been previously, and though Sarandon is never entirely believable in her gray wig, she gives Pearl a credibly troubled center. Apart from Bates, who brings her savvy brand of straight-talking aplomb to Lenore, the rest of the cast is pretty much wasted, with the talented Collette given barely anything to do in a throwaway role. And Falcone’s direction is at best blandly workmanlike, apparently designed to give McCarthy and Sarandon as much leeway as possible. Technically the movie is a passable package, but without any convincing sense of place except for the final shots at the Falls.

“Tammy and the Bachelor” scored a couple of sequels, and even a television series. “Tammy” is likely to do neither.