This, the 3rd "Wimpy Kid" movie, is about the adventures of a middle school kid named Greg Heffley (Zachary Gordon) and his best friend Rowley Jefferson (Robert Capron). The movie opens on Memorial Day weekend, and the Heffleys are headed for the municipal pool for the day. Greg Heffley is annoyed because it's hot, crowded, and the little kids are peeing in the pool. They run into a kid that Mr. Heffley (Steve Zahn) recognizes as a former neighborhood troublemaker. He is now lifeguard and a straight arrow who credits his transformation to his parents sending him to Spag Union, a boys' boarding school for slackers and troublemakers. We can see a light go on over Mr. Heffley's head, and Greg is very worried.
It's the last day of 7th grade for Greg and Rowley. Greg is desperate to have his pretty crush, Holly Hills (Peyton List), sign his yearbook. He asks her to add her phone number to the autograph, but the final bell rings just as she is writing, and she gets swept up in the crowd racing for the door before she can finish. Outside the school, Greg is just about to ask Holly to finish writing her phone number, when Holly's beautiful but bratty older sister, Heather (Melissa Roxburgh), arrives to drive Holly home. She is very rude and dismissive of Holly and her friends and has no time to talk as she is busy planning her sweet 16 party. Greg is disappointed that he is not able to get Holly's number.
Greg narrates that he plans to spend the whole summer watching TV and playing video games. He says that his father would rather that Greg be like the Warren boys from across the street who spend the summer outside playing sports, but Greg says that he is an indoor kid who just likes video games. He says that he and his dad have very little in common except a mutual hatred of the unfunny, cutesy "Li'l Cutie" comic strip that luckily is coming to an end that summer, as the artist is retiring.
On the first day of vacation, Greg spends the entire day playing video games until just before his dad comes home from work. He goes outside and runs through a sprinkler, so when his dad comes home, he tells him that he's sweaty from playing football outside all dad. Mr. Heffley is very impressed until he turns on the TV and sees it is in video mode. He decides to unplug the TV and game console and tells Greg he is banned from video games all summer. Mrs. Heffley (Rachael Harris) pulls Greg's dad aside and suggests to him that if Greg is going to be banned from TV that he and Greg need to spend time together to bond. There is a montage where Greg and his dad try to bond. Mr. Heffley takes Greg fishing, but he gets freaked out by the worms and accidentally dumps the entire bucket of bait into the pond before they can even start. They then go to one of Mr. Heffley's beloved Civil War re-enactments. Greg is forced to dress up as a Union Army bugle boy, and he is clearly bored and embarrassed. In his boredom, he starts playing the bugle, and inadvertently alerts the "Confederates" to their position, and the whole battle re-enactment is ruined. Mr. Heffley gives up on bonding with Greg.
Rowley calls Greg to ask him if he wants to spend the day at the Country Club where he and his family are members. Greg is completely amazed when he walks in the club. It is beautiful. The water is clean, and the pool isn't crammed with people. He and Rowley run into Holly Hills on the tennis court. She and her family are members, too. Greg tells her that he and Rowley are "all about" tennis, so she invites them to play with her the next day. He and Rowley show up the next day, and to Greg's horror, he sees Holly's doubles partner is Greg's nemesis, Patty Farrell (Laine MacNeil). Patty, of course, takes great pleasure in humiliating and destroying Greg and Rowley on the court. After the game, Holly realizes Greg and Rowley have never played tennis before. Greg says it looked so much easier in Wii Tennis. Far from being annoyed, Holly is flattered that the boys would go to such lengths to impress her, and she offers to help them improve their game, much to Greg's delight. At home that night, Greg's dad tells Greg that he has gotten him an unpaid internship at his office, and he and Greg can spend the whole summer together at work. Greg thinks quick and tells his dad that he saw a "Help Wanted" sign at the club, and he is going to be working there for the summer. Greg's dad is very impressed.
The Heffleys go to a 4th of July BBQ at the Warrens' house across the street. Mr. Warren is a big, overgrown bully who was a member of Mr. Heffley's Explorer Scout troop when they were kids. Mr. Warren says he remembers Mr. Heffley being a big crybaby -- even though he was crying because he broke his arm in two places. Mr. Warren suggests that Greg's dad put him in Explorers so he will turn out like a real man. At the BBQ, Rodrick (Devon Bostick) tells Greg he doesn't really believe that he is working at the club. Greg admits he lied, and Rodrick agrees not to tell their dad if Greg agrees to help sneak him into the club so he can get near Holly's sister, Heather. Greg has no choice but to agree.
The next day, Greg sneaks Rodrick in the service entrance of the club. He immediately starts ordering food on the Jeffersons' bill. He sees Heather in a lifeguard stand, and he hatches a plan. He jumps in the pool and pretends to drown, but Heather is too busy texting on her phone to even notice him. A fat, hairy man "rescues" Rodrick and gives a disgusted Rodrick mouth-to-mouth.
Greg's dad has now enrolled him in Explorer Scouts in a troop with Fregley, Chirag, and Rowley. Greg's dad has volunteered to be assistant troop leader. Greg is utterly hopeless at the scout meeting. He isn't really interested and can only carve a hole in piece of wood, while Chirag carves an intricate Indian goddess statue. Greg begins to worry that his dad will send him to Spag Union. He and Rowley discover a Spag Union informational DVD that Mr. Heffley has ordered. They are horrified to find that Spag Union is more like a prison where the boys have no fun and no freedom.
Mr. Heffley might not be ready to send him to Spag Union, but he decides that Greg needs more responsibility, so he gets the family a dog. Greg wants to name the dog "Killer" or something tough, but Greg's little brother, Manny (Connor and Owen Fielding), decides he should be called "Sweetie." Greg doesn't exactly bond with the dog. He hates scooping the poop and how the dog is constantly slobbering and stealing all his food from the counter. One day before dinner, Greg and his dad see that the dog has ahold of Manny's beloved blankie. They work together trying to get the blankie away. Greg distracts him by waving a roast beef his mom had left on the counter to rest while Greg's dad grabs the blankie. They get the blankie back, but the dog grabs the roast and gets his nasty slobber all over it. They get the roast back, and Greg's dad tries to salvage the roast by washing it off. At dinner, Rodrick raves about how unusually delicious the roast. Greg and his dad refuse to eat any and say they are full of vegetables.
Rowley invites Greg to spend the weekend at the beach with him and his family. Before he goes, Greg's parents give him a cell phone, but he is disappointed to see that it is a "ladybug" phone that can only call home and 911. Greg looks forward to a weekend of fun, but the Jeffersons' beach house turns out to be on the quiet, isolated beach far from the boardwalk. Rowley's parents agree to take them to the boardwalk. Mr. Jefferson (Alf Humphreys) asks each person what kind of ice cream they would like, and they all shout out their choices. Mr. Jefferson returns with one cone with three scoops, and they all are supposed to take turns licking it. A grossed-out Greg refuses. Rowley's parents tell the boys to meet back up in an hour, but they are not allowed to ride any scary rides. Rowley and Greg spend the evening playing arcade games and riding scary rides. They are one hour late meeting up with Rowley's parents, who tell Rowley that they are very disappointed in him. Rowley is upset, and Greg doesn't understand why. They got off without punishment, but Rowley says his parents' disappointment is the worst punishment at all.
That night, Greg and Rowley are forced to share a twin bed to sleep in. Greg sneaks downstairs after Rowley falls asleep and sees Mr. Jefferson on the computer. When Mr. Jefferson gets up, Greg sneaks over to the computer and writes an email saying "Help, get me out of here. These people are driving me crazy." He means to send the email to just his mom, but he accidentally sends it to Mr. Jefferson's whole address book. In the morning at breakfast, Mr. Jefferson is surprised to find that he has over 900 emails. Greg realizes what happened, and races upstairs to pack his bag. He tries to call his parents on his ladybug phone but accidentally calls 911 instead. When he comes downstairs, the Jeffersons have figured out that Greg sent the email and are very upset. The police come to answer the 911 call, and Mr. Jefferson ends up getting wrestled to the ground and handcuffed. Greg's dad comes to get him and comments to Greg on the way home that Rowley must be a bad influence because Greg always seems to get in trouble when he's around Rowley.
On Monday, Rowley calls Greg to tell him that they won't be going to the club that day. Greg decides to sneak in himself, and he is surprised and hurt to see Rowley there. Rowley admits that he lied. His parents told him that they don't want him inviting Greg to the club anymore, and Rowley didn't want to hurt Greg's feelings. Greg forgives Rowley, and things continue as usual. Since Greg isn't allowed to go with the Jeffersons anymore, he has to sneak himself and Rodrick in. He turns out to be quite good at the scam and winds up pulling the wool over eveyone's eyes. He spends the summer living the good life. At the club one day, he runs into Holly and Heather, who is in a bad mood because the DJ for her sweet 16 party cancelled. Greg sees a chance to spend more time with Holly and score points with Rodrick, so he tells Heather that he and Rowley are roadies for a band that would be able to play at her party. She agrees, and Rodrick is thrilled at his chance to play at Heather's party.
Finally, Greg's bubble is burst when Greg's dad wakes him up one morning and tells him that he is going to drive Greg into "work" at the club. Greg's dad drops him off out front of the club, and when Greg walks in, he is horrified to see Mr. Jefferson arguing at the front desk about his bill. He insists that he never ordered $250 worth of smoothies. The front desk clerk says it was his "son" who ordered them, and she points at Greg. Greg knows he is busted, but things get worse when Greg's dad walks in, saying Greg forgot his sunscreen. Mr. Jefferson angrily tells Greg's dad that Greg has run up a huge bill, and Mr. Heffley says that is impossible because Greg works there. The club manager says Greg doesn't work there, and they never hire minors. Greg's dad is devastated by Greg's lie, and he quietly agrees to pay the bill. On the way home, Greg keeps waiting for his dad to yell at him, but Greg's dad says he isn't mad, he's just disappointed, and Greg finally understands why those words are so awful.
That weekend is the Explorer Scout campout, and Greg's dad is still upset about Greg's lie. He can barely look Greg in the eye. The troop picks a prime camping location, but they are chased away by Mr. Warren and his troop of bullies. Greg's troop is forced to camp out next to the latrines where they build a bonfire and tell ghost stories. Greg's dad is hurt that Greg doesn't take the whole thing more seriously. In their tent that night, Mr. Heffley yells at Greg and says that he makes mistakes, too, but he always owns up to his mistakes, and Greg never does. The next day, Greg and his troop are walking through the woods when they overhear the other troop say what losers the kids in Greg's troop are. Mr. Warren says they can't help being losers because their leader, Mr. Heffley, is a loser. Greg is hurt to hear his father insulted, and he and the troop plot revenge. They build a web of string between two trees and cover it with honey. They will put ants in the other troop's sleeping bags. When the kids run out of their sleeping bags, they will run into the tree and get caught in the sticky web. Greg sneaks into Mr. Warren's tent to put the ants in the sleeping bag, and he is shocked to see that it is filled with a microwave, TV, fridge, and all the conveniences of home. He hears Mr. Warren coming, so he hides under a cot, but the ants escape from his pocket and start crawling all over him. He starts screaming and runs away before Mr. Warren can spot him, but Mr. Warren knows it was someone from Greg's troop. Mr. Warren confronts Mr. Heffley and his troop and demands answers. No one in the troop will confess until Greg decides to step forward and admit responsibility. The other boys won't let Greg take the whole blame, and they say they only did it because Mr. Warren had insulted Mr. Heffley. Greg's dad is pleased that Greg admitted his wrongdoing and that he was sticking up for him. He calls Mr. Warren out at as cheat for having a TV, etc. and decides not to punish the boys. He and Greg spend the night at a motel, where they eat pizza and watch TV in bed. Greg's dad confesses that he hates camping, and he tells Greg that he has no intention of sending him to Spag Union. They see on TV that the Li'l Cutie comic isn't ending after all -- the artist's son is going to continue the strip. Greg and his dad pretend to be upset, but they are both glad that they actually do have things in common, like their shared secret about the roast beef and their dislike for the comic.
Summer is almost over, and it is time for Heather Hills' party. Rodrick is very excited and wants to make a good impression on Heather. Greg and Rowley are there acting as roadies for Rodrick's band, and Holly is glad to see Greg. The party is very fancy and dignified. Everyone is in formal clothes, and there is a giant ice sculpture of the birthday girl in the middle of the tent. Rodrick's band starts a horrible, punk version of a Justin Bieber song hoping it will impress Heather, but she is horrified. Rodrick jumps off the stage and winds up bumping into Heather. She knocks over the giant ice sculpture, causing a domino effect. The food goes flying, including a chocolate fountain, which lands all over the guests. Greg thinks Holly will be upset with him, but she tells him that she is thrilled that her sister is getting her comeuppance. She slips her hand into Greg's hand, and they stand hand-in-hand laughing and watching the comical chaos. Greg realizes that things are pretty OK, and that the summer will end on a high note.
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