In the pantheon of recurring Jackass sketches ripe for full-length cinematic treatment, Johnny Knoxville and Spike Jonze’s prosthetically-aged, lecherous old men seem like an odd choice for the full-length treatment. It might be that Irving Zisman, Knoxville’s alter ego and the protagonist of Jackass Presents: Bad Grandpa, is one of the only spinoff possibilities that doesn’t involve physical mutiliation. Or it might be the world’s perennial affinity for hidden-camera pranks, the kind that make you wonder exactly how these crazy guys keep pulling it off. Whatever it might be, Bad Grandpa is definitely one thing for sure: surprisingly entertaining, more so than it really has any reason to be.

Like Borat sans the biting social commentary, Bad Grandpa follows Zisman around with his grandson Billy (Jackson Nicoll), a precocious 8-year-old who tends to take everything in stride. Nothing gets to Billy, not even his mother’s incarceration on drug charges or his burnout father’s evident disinterest in him. When his mother is put away, the recently bereaved Zisman is tasked with taking Billy from Lincoln, Nebraska to Raleigh, North Carolina to be with his father. The two boys of different ages throw Zisman’s deceased wife in the trunk of his Cadillac, and take to the road. All this, though, is preamble to the real point of Bad Grandpa, which is to say the comedic pleasures of watching an old man and a young boy cuss, drink, and occasionally get their genitalia stuck in vending machines, as terrified strangers are tricked into looking on.

Like any of the Jackass movies, Bad Grandpa flings all kinds of shit at the walls (on one occasion, literally) to see what sticks. And like many of them, it’s a mixed bag of a comedy, with more than a few bits falling flat throughout. As the harassment of strangers goes, the film goes to that well early and often, and it’s a little too often that the film relies on its standby gags of the horny old man and the lonely young boy getting people to react to them. Bad Grandpa works the best when a little planning is involved; the funniest setpiece in the entire film surrounds Zisman’s trip to a male strip club on ladies’ night, where the joke isn’t just an old man doing stuff but instead a) how weird the club turns out to be and b) how far Knoxville takes the joke with an unsuspecting, increasingly hostile crowd as his marks. Likewise, Irving and Billy’s late-game trip to a child beauty pageant is both mortifying and side-splitting.

What’s more interesting about Bad Grandpa is how often Knoxville and Nicoll manage to extract genuine sweetness and affection from what’s ostensibly a gimmick movie. Though their backstories really only exist as framing devices to move the film between pranks, there are moments in which Zisman’s aging nostalgia for his youth or Billy’s pleas to stay with the old man actually hit on something moving. You’ll see the dangling (fake, mercifully) testes and the dirty wordplay coming from a mile away, but the warm heart at the center of Bad Grandpa is a lot less likely. When the film seems like it’s running out of comic mileage from its high concept, and there are a few moments in which that seems inevitable, it’s that heart that keeps Bad Grandpa from becoming exactly what you’d probably think it would.