Seeing Melissa McCarthy's (2013's "The Heat") rise from a steadily working character actress to an Oscar-nominated box-office draw and leading lady has been one of the most inspiring Hollywood success stories in memory. Far from some 20-year-old starlet who got lucky with her looks and an audition, McCarthy has paid her dues (and then some), exhibiting time and again what a rangy, effervescent scene-stealer she can be. Now an Emmy-winning performer on CBS' long-running sitcom "Mike & Molly" as well as an in-demand actress of the silver screen, McCarthy comes off in real life as kind, generous and grateful, not for one second taking her good fortune for granted. In other words, she has earned everything she has gotten in recent years, and deserves it. That New Line Cinema agreed to fund her screenwriting debut and hand directorial duties to husband/co-writer Ben Falcone is a true testament to the studio's trust in her drawing power. "Tammy" probably would not have gotten made without McCarthy on board, and whether the finished film is great, so-so or an abject failure, what cannot be taken away is McCarthy and Falcone's achievement in creating and seeing through to fruition a passion project that has their names written all over it.

Tammy's (Melissa McCarthy) day has gone from bad to terrible to worse. One minute she's driving down the road jamming out to The Outfield's '80s hit "Your Love," and the next she has hit a buck and come close to totaling her car. Late for work, she is promptly fired from her piss-poor job at fast food restaurant Topper Jack's. Arriving home early, she walks in on husband Greg (Nat Faxon) having a romantic dinner with Missi (Toni Collette). As Tammy is apt to do when things get tough, she threatens to leave town for good—but usually returns the next day to the unhappy life she's been leading all along. This time is different, though, as diabetic, hard-drinking grandma Pearl (Susan Sarandon) packs her bag and insists on riding shotgun. Headed for Niagara Falls but taking a wrong turn to Louisville, Kentucky, Tammy and Pearl find themselves attempting to work out the differences they've had in the past while prompting each other to take a long look at where they're at and what they want from their futures. If they can avoid getting in trouble with the law long enough, they might have a sporting chance, too.

A road comedy where the travelers accidentally drive in the opposite direction and then end up staying put for a while in their unintended location, "Tammy" emulates the characters' trajectory by seeming to run in place for most of its length. As a protagonist, Tammy is a complicated individual: straightforward, prickly, conflicted, big-hearted. In one scenario, she forcefully hits on Bobby (Mark Duplass) moments after meeting him at a BBQ joint, and later on she is pulling away from his flirtations and confessing she's still technically married and has a lot of emotional baggage she must first deal with. A loose cannon who isn't above crashing a jet ski or robbing a Topper Jack's with a paper bag in the shape of a gun, Tammy is still friendly enough to bond with the employee she holds up, Becky (Sarah Baker), and makes sure to return the money when she no longer needs it to bail her grandma out of jail. In other words, Tammy is a mess, but she's a likable mess. The film she is in, however, is not nearly as original or confident, a familiar concept that combines broad humor with heartfelt pathos in the same way 2013's "Identity Thief" did. At a certain point, the viewer wishes Tammy and Pearl would keep physically moving while more succinctly exploring the lingering, passingly hinted-at conflicts permeating between them.

Melissa McCarthy and an aged-up, gray-haired, swollen-ankled Susan Sarandon (2014's "Ping Pong Summer") are an effortless pair as contentious familial road buddies Tammy and Pearl. Both women are flawed—Tammy tends to act before thinking, and Pearl has burned a bridge or two with her alcoholism—but one also sees in them the potential to change their ways for the better. Fortunately, when it comes to cinematic road trips starring Sarandon, things end more hopefully than they did for her Louise and Geena Davis' Thelma. The supporting cast is filled out by an embarrassment of riches, but only some are used to their strengths. Mark Duplass (2012's "Your Sister's Sister") gives Bobby a believable, affable sincerity, his chemistry with Tammy softening as they get to know each other better but readily apparent from the first scene they share. Kathy Bates (2011's "Midnight in Paris") has some nice moments as the tell-it-like-it-is Lenore, a lesbian cousin of Pearl's whom they stay with just in time for a 4th of July celebration. Dan Aykroyd (2007's "I Now Pronounce You Chuck and Larry") is in and out too quickly as Don, Tammy's dad, but is a welcome face in his small part, while Sarah Baker (2012's "The Campaign") shows a memorable comedic spark as put-upon Topper Jack's employee Becky.

The other actors are underutilized to the point where the only explanation for their involvement is that they are friends of McCarthy and Falcone. Allison Janney (2014's "Bad Words") gets very little to do as Tammy's mom, Deb, looking perpetually concerned. Sandra Oh (2010's "Ramona and Beezus") plays Lenore's partner, Susanne, given nothing to say other than the sporadic chime-in, usually within a group of voices. As Missi, the other woman whom Tammy's husband has started dating, Toni Collette (2013's "The Way Way Back") is almost offensively wasted in a nothing role. The three times the narrative returns to Greg and Missi, they are sitting down to a meal at home, leading one to wonder if these one-note characters do anything else but nibble off plates.

As much as Melissa McCarthy is a presence worth watching in almost anything, it is difficult to get too excited about the low-key, wishy-washy "Tammy." At least one scene where she makes a move on two guys having dinner at a restaurant to prove to her grandma that men find her irresistible is particularly misguided. When they rebuff her advances, she tells them off and returns to her table. Upon discovering they are gay, she tries to apologize by yelling across the room that she supports their "lifestyle choice." Had this statement led to Tammy being schooled about how being gay isn't a choice at all, it would be fine. Regrettably, the scene ends with her uninformed comment, and will leave some viewers with a bad taste in their mouths. This troubling moment aside, the film means well, but director Ben Falcone seems unsure of what to do with the road-movie formula without falling into been-there-done-that territory. Occasionally worthy of a chuckle and at other times delicately moving, "Tammy" nonetheless strikes as a meandering ride to some fairly pedestrian subjective destinations.