Irving Zisman (Johnny Knoxville), aged 86, walks up to a seedy gas station vending machine, unbuttons his pants, and surreptitiously inserts his penis into the change return slot. After relieving himself, he discovers, to his horror, that he has become stuck within the enclosure. Initially calm, Zisman soon begins to panic and screams for help. Paralyzed patrons, loiterers, and attendants all watch with horror as Zisman leans backwards, his member stretching to cartoonish lengths as he wails in pain. So goes one of the more memorable scenes in Knoxville’s new film, “Jackass Presents: Bad Grandpa,” a simple, loosely plotted road-trip film in which Zisman, an astonishingly crude octogenarian, transports his equally distasteful grandson from his druggie, jail-bound mother in Nebraska to his white trash father in North Carolina. The film, which mixes fictional narrative with real-world reactions to stunts, is a pleasant, oddly relaxed tale that charms its way into the heart and funny bone through the infectious performances of its two leads and the continued ingenuity of Knoxville’s notoriously nasty exploits.

Knoxville’s Zisman, an impeccably costumed, gravelly voiced recent widower, provides an ideal dramatic vehicle for Knoxville. Too often boxed in by his macho “Jackass” persona, Zisman allows Knoxville to explore both the limits of his crudity and the vulnerability of being elderly. Often repulsive in his delusional womanizing, alcoholism, and extreme disregard for his grandson, Knoxville resists making Zisman a complete caricature through sudden, uncanny flips into tenderness. At his wife’s funeral, Zisman delivers a tearful sermon about how he wished he had been less combative. Knoxville’s mastery of elderly vocal patterns shines as Zisman begins to break down. Moments later, however, his wife falls out of the coffin and Zisman attempts to put her back to no avail as the horrified, clueless crowd screams in disgust. Jackson Nicoll matches Knoxville’s impressive performances as Zisman’s grandson Billy, an overweight  eight-year-old whose world-weariness and honesty are both hilarious and touching.

The intermingling between script and real-life stunts works surprisingly well, as the inevitable budding friendship between Zisman and Billy occurs against the backdrop of increasingly ridiculous hijinks. The two giggle and bond as they attempt to pick up overweight and extremely uncomfortable women, crash through windows, and poop on restaurant walls. The script, a joint effort between director Jeff Tremaine, Spike Jonze, and Knoxville, strongly evokes “Borat” in its combination between a buddy film and a probing into the American populace’s reactions to absurd situations. Zisman and Billy visit similarly trashy American haunts (bingo halls, strip malls, grimy corporate restaurants, beauty pageants) to those that Sacha Baron-Cohen highlighted in “Borat,” wreaking havoc on the often-depressing characters found within.

Where “Borat” focuses on the ignorance, extremism, and xenophobia of real people’s responses to stunts, however, “Bad Grandpa” solely shows their shock. The stunts in “Grandpa” are often offensive and hilarious, but seldom provoke any reaction deeper than instinctive disgust. The results are usually funny enough to make up for the redundancy of people’s horrified reactions, but occasionally the stunts seem borderline cruel and pointless without a larger political agenda. The road trip through the Midwest and South semi-coherently explores working-class apathy and cluelessness, but doesn’t drive the point home in any meaningful way. The film would be stronger if the writers had made more of an effort to tease out some ridiculous and politically significant responses to match the stunts.

The film’s ideological shortcomings, however, are made up for by its even and relaxed tone. While “Borat” and other films where plot and real life mix rely heavily on overwhelming peaks and valleys in scripted intensity to retain audience interest, “Bad Grandpa” stays relatively consistent in tone. Nothing really changes over the course of the film, and the predictable and understated denouement actually feels like a relief given the typical reliance on bombastic endings. The resistance to extremism allows for the focus to lie entirely on the effective banter between Zisman and Billy and the creativity and crudity of the feats—and really, why else do people watch “Jackass” films?