The Heffley family: Greg (Jason Drucker), Rodrick (Charlie Wright), Frank (Tom Everett Scott), Susan (Alicia Silverstone), and Manny (Dylan and Wyatt Walters) - plus Rowley (Owen Asztalos) arrive at Corny's, a family-friendly restaurant that Greg thinks is for babies. At the table, Greg and Rowley watch a video of their idol, internet celebrity Mac Digby (Joshua Hoover), whom Susan considers a bad influence. Manny then gets stuck in the tube maze above the ball pit, which means Greg must go get him. He gets bombarded by other smaller kids and then loses Manny in the ball pit. Greg scrambles to find him but then ends up getting a dirty diaper stuck to his hands. As he tries to shake the diaper off, other kids and adults start laughing and recording Greg on their phones.Greg becomes a viral sensation known as "Diaper Hands", which has left him humiliated. He finds out that Mac is going to be at a video game convention, Player Expo. Since the Heffleys are heading on a road trip for their Meemaw's 90th birthday, Greg tries to find a way to get to Player Expo and take a video with Mac so that everyone can forget about Diaper Hands.The Heffleys pack up for their road trip, but their van becomes cluttered with so much luggage that Frank considers flying. Not wanting to miss the convention, Greg suggests putting all their luggage in Frank's boat so that they'll have space and possibly get the boat on the water.As the family hits the road, Susan forces her sons to give up their phones so that they can have a "disconnected" family vacation free of electronics. Greg and Rodrick reluctantly hand over their phones. She also makes them eat healthy snacks instead of stopping to eat something. For fun, Susan pulls out a card game where people confess to things. It takes a real turn when Greg uses something he knows Rodrick did and makes him admit it. Frank and Susan follow suit and make Rodrick admit to other naughty things he's done, then forcing him to apologize on the phone to their neighbor whose house he TP'd a while back. Rodrick shoots Greg a death glare and promises to get him back.The family passes by a grocery store and drops Greg and Rodrick so they don't wake Manny from his nap (because that would be really bad). Instead of sticking to Susan's list of healthy junk, Rodrick grabs a bunch of sugary junk.The Heffleys stop at a crummy motel for the night. The boys want to use the pool but find that it has been drained. All that's left is a hot tub which is being occupied by a family that Greg refers to as "the Beardos" (because the dad has a big beard). Later that night, the family leaves the hot tub, so the brothers go for a dip. Greg has taken his phone, and Rodrick finds out. He goes to tell on Greg as payback for the card game. Greg tries to get out but accidentally pulls a bag of cheese puffs into the hot tub, turning him orange. He then tells Rodrick about the Player Expo and how he may have a chance to earn a cash prize to fix his van and maybe even meet hot girls in cosplay. Rodrick agrees to join Greg.When Greg goes back to his room, he finds Frank using his computer to do work stuff. They agree to keep their electronics use a secret from Susan to not upset her. Greg then tries to go to bed but has no room on the couch or bed. He goes to sleep in the closet but then hears a banging noise from outside. Greg steps out the room and finds the Beardo kids making noise as they've stolen a snack cart. Greg tells them to be quiet but the kids push the cart at him. The cart hits the Beardo family's van, just as Mr. Beardo (Chris Coppola) steps out. He freaks out when the cart scratches the van, and his daughter pins the blame on Greg. Greg runs away until the coast is clear so he can return to his room. In the morning, he realizes he walked into the Beardos' room and slept next to Mr. Beardo. He runs out and back to his room.The Heffleys go back on the road and stop at a country fair. Greg and Rodrick eat deep-fried butter on a stick, but Greg sees the Beardo family walking around. He runs and ends up having to take Manny while Frank makes a business call. The boys come across a game where they must guess the weight of a pig in exchange for a lollipop. Manny manages to guess the exact weight of the pig and earns a small piglet as a prize. Greg says they can't keep it for reasons that unintentionally insult the farm folks in that area, so the boys leave. Greg and Mr. Beardo spot each other, so Greg gives Manny the piglet and runs again. Greg and Rodrick go on a Gravitron, after Rodrick has eaten his 9th butter stick. Mr. Beardo follows Greg in there as the ride starts up. Greg climbs over some kids while Beardo tries to reach him. Beardo presses on Rodrick's stomach, causing Rodrick to blow chunks that fly into Beardo's face (and mouth). The boys run and find the rest of their family, which leads to Susan finding out about Frank and Greg using their phones. She angrily tells them they are leaving and keeping the piglet.Frank is making a call on the road but keeps getting interrupted by his family making noises. Things get chaotic when they nearly get run off the road by a truck driver, causing Frank to drop his phone and accidentally blurt out that he's taken time off, which he didn't tell his bosses about.The family stops at another hotel and sneaks the piglet in a cooler. He ends up eating everything in the minibar, which angers Frank because it means he has to pay extra. When they hit the road again, Greg has to pee and can't wait because the next stop is 37 miles. He goes in two lemonade bottles moments before the piglet starts pooping in the car, to the family's disgust. They have no choice but to drop the piglet off at a petting zoo, to Manny's sadness.The family stops at another hotel but are now closer to Meemaw's party. However, Greg and Rodrick sneak out and pretend to go look for a gift when they are actually going to Player Expo. Greg is excited to be there, except for the fact that someone is cosplaying as Diaper Hands. Greg sees Mac Digby playing a game trying to make "gamer history" by beating the score. Greg sneaks past security and tries to get Rodrick to take a video. Greg startles Mac and accidentally disconnects the game, ruining Mac's score. To make things worse, Frank and Susan show up after seeing the boys on the news. Susan angrily makes her way to the stage and outs Greg as Diaper Hands. Mac and the rest of the audience get excited but Greg is once again embarrassed, knowing he will never live this down. He snaps at Susan that she's been making everyone do what she wants but doesn't care about what everyone else wants. To top it all off, Greg used the front-face camera to take the video, so Greg has no proof of being next to Mac.While back on the road, the tarp covering the boat undoes itself, causing the family's luggage to spill all over the road. The Beardo family shows up and takes advantage of this to get payback on the boys. Beardo steals a bag that contains a scrapbook that Susan wanted to give to Meemaw. The Heffleys go after them but get stuck in traffic. They try to take another route, and they end up overheating their car. Everyone becomes upset over how everything has turned out. The boys make peace with their parents and plan to turn the heat up in their car to get it running again.Frank gets the car running again but they end up in the woods near some hunting grounds. A pack of cinnamon rolls falls near the heater and bursts, startling everyone and spraying goo everywhere. Rodrick freaks out and thinks he's been shot and his brains are coming out, but they realize what the problem is. The car also gets stuck in a small hole. Frank hits the gas as his family steps outside, spraying all of them with mud.The family keeps moving and passes by a motel where they see the Beardo family van. The Beardos go to the pool, giving the Heffleys a chance to sneak into their room, clean up, and get their stuff. Greg gets stuck in the bathroom as Mr. Beardo returns. He stays long enough to poop (and not wash his hands). Greg tries to get out but accidentally turns on the shower. Beardo comes back and tries to get Greg (in a parody of "Psycho"), but the boy squirts shampoo in Beardo's eyes. Greg runs for it and rejoins his family as they got their stuff back. Frank then throws Beardo's van keys into the dumpster as they drive off. Beardo hops in the dumpster and gets a diaper stuck on his hand.The Heffleys run out of road to drive on and start riding up a hill, but their car finally dies. They seem to give up until Greg sees Meemaw's house down the hill. The family starts to push the van down the hill before hopping in. Greg misses the van and hops on the boat. The boat unhooks from the car and goes down the hill, sending Greg flying over the fence and into Meemaw's pool. The rest of the family shows up, and Meemaw is happy to see them.After the party, the Heffleys head for home, but their car breaks down again. A tow truck driver stops on the road, but he only speaks Spanish. Thankfully, Manny speaks good Spanish and tells the driver they need a lift. Manny guides the driver to the petting zoo where they get the piglet back.Greg narrates that things worked out well for everyone when they got home. Frank is allowed time off from work to spend with his family. Rodrick gets money to repair his van. Greg says that while this vacation was far from perfect, it was certainly eventful. He expresses excitement for where they'll be going next year, but Susan steps in to say they will be flying.