The Jackass franchise is growing old gracefully — as much as any movie that begins with an elderly man getting his penis caught in a soda machine can be considered a maturation.

"Jackass Presents: Bad Grandpa" takes the old man-prosthetic-makeup pranking antics from the previous three movies to feature-length extremes, with Johnny Knoxville unleashing his 86-year-old Irving Zisman character on unsuspecting crowds. The uneven result is definitely not for prudish moviegoers, is definitely funny for everyone else, and even approaches poignancy in one or two scenes.

‘JACKASS PRESENTS: BAD GRANDPA’ Rated: R Opens today

The biggest shock: This time there’s actually a plot. Granted, it’s a plot that includes a prosthetic hanging scrotum and explosive diarrhea gags. Still, "Jackass" is so much closer to "On Golden Pond" than anyone could have imagined when the MTV show premiered 13 years ago.

Knoxville returns mostly as a solo act, with longtime director Jeff Tremaine and co-conspirator Spike Jonze working behind the scenes. The production feels diluted in the beginning, like a 1970s rock band that’s touring with only one of the original members. Adding to the shaky foundation is a new format, which mixes "Borat"-style hidden camera infiltrations with scripted interludes — following a horny old coot (Knoxville, buried in convincing prosthetics) on a road trip to deliver his equally profane grandson to a no-good father.

Making up for the loss of the rest of the Jackass crew is 9-year-old sidekick Jackson Nicoll, who stays clear of the higher-profile stunts — Knoxville gets catapulted through a plate-glass window — while holding his own interacting with the public.

Nicoll’s comic timing is impressive, especially during a series where he asks strangers if they’ll be his dad. The powerhouse finish involves Nicoll’s participation, in drag, in a little girls’ beauty pageant in North Carolina.

The "Jackass" filmmakers once again push comfortable boundaries for moviegoers, this time with a child involved. The language coming out of the boy’s mouth on its own would earn the film an R rating. But "Bad Grandpa" feels less depraved than the previous entries. For one thing, they appear to use fake excrement in their pranks instead of the real thing. (Baby steps.)

LATER, as Knoxville and Nicoll continue to do the wrong thing, the people they encounter in rural pockets of the Midwest and South obey their moral compasses at almost every turn. The third act includes a drop-off with a group of bikers who act as guardian angels for abused children. Their tenderness toward the neglected child they think has entered their bar is actually kind of moving.

Some of the pranks are masterfully executed; the beauty pageant and a disastrous funeral near the beginning stand out. But on the whole, "Bad Grandpa" can’t locate a consistent groove. Even as Knoxville stays in character, the filmmakers never find the right pacing between the prank-driven scenes and often awkward narrative elements. Meanwhile, curiosity builds for moviegoers, who are left wondering how the "victims" reacted after each prank was revealed.

Knoxville, Tremaine and Jonze give us that pleasure in one big dump during the final credits, which shows that Nicoll’s parents were indeed on the set, and that the bikers in particular took the joke really well. Your faith in humanity will be restored, even as they clean the fake poo off the wall.