“Tammy,” from a screenplay written by Melissa McCarthy and her director-husband Ben Falcone, was intended to be a starring vehicle for the gifted comedian. However, miscalculations ripple through McCarthy’s rambling shtick, probably because, without someone to reign her in, she goes over the top and takes a long tumble down the other side.

The film does deliver a few laugh-out-loud moments, especially when McCarthy uses her generous proportions as a tool to enhance her physical comedy. The best example finds her making several attempts to jump onto the counter of a fast food restaurant.

When we first meet Tammy, she’s dressed in a red and yellow fast food restaurant uniform, driving a beat up Toyota down a lonely highway, singing and acting out the song (hands on the steering wheel be damned) while reaching into the back seat, where she’s left a favorite treat. A moment later she’s hit a buck deer, receives a gushing bloody nose and all but demolishes her car.

Tammy arrives late to work, lucky to get there at all, in her smoking heap. Clearly, Tammy’s dictatorial boss (played by McCarthy’s husband Falcone), has eagerly anticipated this day and fires Tammy on the spot. He demands the return of her “badge,” a cheap plastic name tag Tammy refuses to surrender, declaring, “You made me pay for this crummy thing. I’m keeping it!”

Caught in a whirlwind of emotions, Tammy relinquishes her employment in a manner many have fantasized about, but most are too sensible to indulge. Licking her hands and touching all the burgers, Tammy whips her hair through the french fries and throws a large box of ketchup packets on the floor, all while telling her boss precisely where he can shove it.

When she returns home, understandably angry and upset, Tammy discovers her husband (Nat Faxon) enjoying a romantic dinner in their dining room with the pair’s daft, pretty neighbor (Toni Collette). Once again there’s no holding back when she lets hubby know exactly what she thinks of him. Tammy gathers up her clothes, stuffs them in an old suitcase and marches out the front door, where the suitcase promptly falls apart, spreading her personal belongings far and wide. Not to be deterred, Tammy regathers her stuff and huffs off toward a house down the street.

The door is opened by Tammy’s mom Deb (Allison Janney). Before the door is shut, Deb is barraged by Tammy’s demands that Deb provide her daughter with money and a vehicle so she leaves town for good. Deb refuses to do either, but grandma Pearl (Susan Sarandon) wants out of Deb’s house. She offers to finance the trip and hand over the keys to her caddy, provided Tammy bring granny along.

Thus begins a road trip that’s more sad sack than mad cap, and one that finds McCarthy plying her comedy in a blathering, unfocused manner. Tammy, a take-no-prisoners and take-no-crap kind of gal, stumbles from one humiliating situation to another, trying to show everyone that she won’t be kept down. It’s an understandable sentiment that might go further if Tammy’s took her own bad decisions into account.

Grandma Pearl is an amorous alcoholic whose vices and libido cause her to frequent bars where she drinks too many cocktails and throws caution to the wind. Fortunately, Pearl catches the eye of Earl (Gary Cole), a nice guy who treats Pearl like a lady even when the pair carry on like rabbits in rut. Meanwhile, Tammy has the hots for Earl’s son Bobby (Mark Duplass), an attractive man who is the only adult in the group.

After getting themselves into a jam, Tammy and granny are rescued by Pearl’s wealthy cousin Lenore (Kathy Bates) and Lenore’s lesbian partner Susanne (Sandra Oh). Ensconced at Lenore’s waterfront mansionette, Tammy and Pearl both begin to see only they can take control of their lives, which means taking ownership of their shortcomings. However, the film fails to persuade us that either woman has put in the work necessary to undergo these epiphanies, which feel both forced and inorganic to their characters.

No one goes to a McCarthy film hoping to see her character clean up or grow up, but it’s where the story takes us. Although McCarthy and Sarandon’s comic stylings are among the best in the business, this underdeveloped material lags behind their abilities, relying on the actress’ charisma to persuade us they have indeed undergone a cathartic experience. Here “Tammy” stops being funny and starts taking itself seriously. It’s a fatal error that keeps us from laughing with, and leaves us laughing at, “Tammy.”