After scoring a breakaway hit with their 2007 film “Juno,” screenwriter Diablo Cody and director Jason Reitman have teamed up once again, this time for “Young Adult.” The film continues Cody’s penchant for writing about high-schoolers, only this time it’s from the perspective of people who graduated but never really left.

Mavis (Charlize Theron), a depressed ghost writer for a hugely popular series of young adult novels, was the prototypical blonde bombshell during her high school days; her ex-flame, Buddy (Patrick Wilson), now happily married with a newborn baby, was her jock counterpart; and Matt (Patton Oswalt) was the resident “theater fag”—an unapologetic nerd who was the victim of a vicious beating by a group of bullies, leaving him partially crippled and with his fair share of scars, both physical and emotional.

Conflict arises when Mavis receives news of Buddy’s newfound fatherhood. Convinced the two are still destined to be together, she leaves her apartment in Minneapolis and returns to her small hometown, hoping to derail his marriage and relive the glory days of high school. In the process, she strikes up a unique friendship with Matt, who also seems stuck in his teenage years.

Though it strives for moments of humanity and pathos, “Young Adult” is clunky and unimaginative, lacking the nuance of Reitman’s previous film, “Up in the Air.” Though both feature stunted protagonists who exist in a sort of self-inflicted stasis, the added presence of Cody, who’s best known for introducing the world to hamburger-shaped telephones, renders what could have been a strong narrative insufferable and pejorative.

The script for “Juno” revealed Cody to be hyper-immature—or, at the most, acutely aware of her own hyper-immaturity, which might actually be worse. That film smothered its lack of insight in goofiness and mall-hipster quirk, making it absolutely impossible to take seriously.

Enter “Young Adult,” a far more cynical take on middle-American life. Though both films are informed by a similar milieu—they’re set in small, Midwestern towns which are populated by an assortment of hapless, dopey people—the presence of Mavis undercuts any and all opportunities for the sort of good-natured mockery found in “Juno.”

To Cody and Reitman’s credit, it’s fairly ballsy to center an entire film on a character as thoroughly detestable as Mavis. There’s nary a moment in which she appears to make any decision that isn’t motivated by self-interest, though there is the occasional allusions to a deeper, internal conflict—which might have read as ambiguous on the page but holds little weight on screen.

“Young Adult” could very well be an elaborate cautionary tale, a means to decry those of us whose opinions of ourselves are far too high. But as the film trudges on, it builds to a half-hearted denouement in which Mavis comes to the kind of realization that even grade-schoolers could recognize as flawed.

Reitman attempts to dig himself out with his usual bag of tricks—handheld cameras, jumpy editing, insert shots of props meant to externalize his characters’ inner-selves—but to diminished returns.

At the end of the day, “Young Adult” is a lazily constructed and poorly executed film, which, if you’re Cody, is par for the course, whereas, if you’re Reitman, is a disappointing step backward.