When the family prepares for a road trip to Indiana for the 90th birthday of Mom's grandmother, Greg devises a plan to undo his internet infamy: He'll get his picture taken with famous gamer Mac Digby (Joshua Hoover) at a video-game convention in Indianapolis. But Greg's plans hit a snag when Mom announces that, in the interest of family bonding, this vacation will be free of electronics — even for Dad, who has hidden the fact that he has to work during part of the trip.
Director/co-writer David Bowers (returning from the series' second and third installments) sets up a series of road-trip vignettes, most of them highlighted by something gross. Besides the diaper, the following substances are deployed: mud, baby formula, human pee, piglet poop, pigeon poop, vomited deep-fried foods, Cheeto dust, exploding cinnamon-roll dough, and probably others I've mercifully forgotten.
All the while, Greg is shown to be a real jerk, a bratty kid who puts his need to avoid embarrassment ahead of his mother's hopes for a happy family trip. The sheer mean-spiritedness of his selfish behavior has always been a problem in this franchise, but this time it's more stark than ever.
Greg's obnoxiousness is not the movie's only sin. There's also the uninterrupted stream of toilet humor, the stupidity of the story, the bargain-basement production, the shameful way it treats the only positive character — Silverstone's loving supermom — as a shrill nag, and the least-appropriate remake of the "Psycho" shower scene ever.
But the worst thing about "Diary of a Wimpy Kid: The Long Haul" is simply this: There's not a single laugh to be had in 91 excruciating minutes. Parents are advised not to wimp out, but to stand strong and reject this kind of soul-crushing moviemaking aimed at their kids.