We begin in 1984 where a young Donny Berger brags to his friends about scoring tickets to Van Halen. One of the friends suggests taking a girl who they think is cute, but Donny declares he's not into girls anymore...he's into women. Cue Ms. McGarricle (Eva Amurri), the smoking hot teacher walking through the halls, attracting the attention of many young boys. Donny makes a very stupid move trying to hit on her by inviting her to the concert and asking for a handjob, resulting in a month's worth of detention.
In detention, however, Ms. McGarricle comes on to Donny, a very obviously inexperienced virgin, and invites him into her office, where his friend can hear the action. We see that this affair continues over several occasions, culminating during a school assembly, where they are caught having sex behind the curtains. As Ms. McGarricle runs away embarrassed, the entire student body (and a few faculty members) begin applauding Donny, then give him a standing ovation as he runs upfront and embraces his victory.
Ms. McGarricle is put on trial, where the judge gives her 30 years in jail because the affair has resulted in a pregnancy. Donny's dad is pissed because the baby will be in his custody. As a result of this scandal, Donny becomes a nationwide sensation, getting a deal for a TV movie and becoming an overall celebrity.
Cut to present day Donny (Adam Sandler) who meets with a man named Jim Nance (Rex Ryan - and yes, the jokes are there for sports fans) who informs Donny that he owes the IRS $43,000 after skipping taxes since 1994. They both talk about Donny's son that he hasn't seen since he moved out, evidently because he was named Han Solo and was incredibly overweight. Jim suggests Donny get him to help, but he doesn't think that'll do anything. Jim also tells Donny about an obese man running in a marathon, and the odds on him are 8000 to 1. Donny then bets $20 on him.
Donny goes to a strip club and talks to waitress Brie (Ciara) and stripper Champale (Luenell). He sees a magazine with his son's picture, now very slim and having changed his name to Todd Peterson, and even making up a story that his parents died in an explosion. Donny learns that his son is getting married and that he is a successful hedge fund manager.
We meet Todd (Andy Samberg) and his fiancée Jamie (Leighton Meester). He is so nervous about her parents coming over that he even has a spare pair of underpants with him. She tells him to relax.
Donny goes to see a guy named Randall Morgan (Dan Patrick) for money. He bumps into Vanilla Ice, who hates Donny for sleeping with his mom. They get into a fight until Randall breaks it up. He meets with Donny privately and tells him that he is no longer relevant like he was when he was a kid, so Donny schemes to get his name back out there while scoring some money. Randall tells him to get his son to come with them to the prison where his mother is for a reunion special, and Donny would get $50,000.
Todd takes Jamie and her family to stay at his boss, Steve Spirou's (Tony Orlando), house. Jamie's brother Chad (Milo Ventimiglia), a marine, comes in and intimidates Todd. As they are having lunch, Donny appears and freaks Todd out. He pretends Donny is his old friend, giving Donny the opportunity to come up with an outrageous story about how he saved his life, which the family seems to believe. Todd pulls Donny into a room to talk to him about how he was an awful dad and to go away so he won't ruin his wedding, but Donny asks Todd to give him a chance, as well as going to see his mother in prison, making up a story about how she is terminally ill. Todd tells him to wear a tie for a cocktail party that's about to happen.
Donny puts on a tie but looks as informal as he can. He does manage to make an impression on Steve and his mother, along with other guests by going "WAZZUP!" He tells the guests about the made-up story about saving Todd. He also meets a co-worker of Todd's named Phil (Will Forte) and his wife (Rachel Dratch). Phil admits that he had no idea why Todd picked him as the best man, so he passes it to Donny. Donny then notices a baseball field nearby and has everybody go play. This makes Todd nervous because he can't play sports, and it shows as he gets hit in the throat during the game.
At night, Donny hears Jamie yelling at Todd to get the cake, showing how whipped he is. Donny sees him in the shower wearing a swimsuit, revealing an old tattoo of New Kids on the Block with warped heads, which Donny finds hysterical. They suddenly get into a tickle fight which becomes awkward when Donny waves his boner in Todd's face. Chad comes in and thinks they're wrestling, so he starts wrestling Todd, only to get knocked out by a bottle. Later, he whacks off to Steve's mom, who used to be a model. He admits to being turned on by the old woman now as opposed to the picture of her younger self. In the morning, she and Todd find all the used tissues on his bed.
Everybody goes to church for a rehearsal with Father McNally (James Caan). He says he is sorry Todd's dad couldn't be there, and Todd says he deserved his imaginary fate of dying in a car explosion. McNally berates him for speaking of his father like that when he didn't abuse him, so Todd insults him. This leads to a fight outside, and Todd is getting his ass kicked until Donny knocks out the priest with a bottle. The family is pissed at Todd for provoking the priest, but Donny saves him by telling the family that Todd had a bad experience with churches after his dad's "death," so he is forgiven, and they decide to have the wedding at Steve's home.
As they are preparing at the home, Donny learns that there will be a bachelor party. It turns out Phil booked a party at a men's treatment spa, bringing Donny, Todd, Chad, Steve, and Jamie's dad Gerald (Blake Clark). Donny hates the place because he is not getting pleasures, even trying to proposition the masseuse (Jackie Sandler). He decides to take the guys to the strip club (not before nailing the hostess (Ana Gasteyer) who, despite her acknowledging how vulgar he is, still wants to screw him), where they all get drunk and get lap dances. Brie appears and tells Todd she knows he is Donny's son, but it'll be a secret.
Chad, Steve, and Gerald go home while Phil gets head from a stripper. Donny and Todd leave to bond. Donny gives Todd an earring that Ms. McGarricle gave him, though he makes him bleed while putting it in. They run into Vanilla Ice (who was known to Todd as Uncle V as a kid), working at a skating rink. Donny apologizes to Ice for sleeping with his mom, and he forgives him. They then go into the rink after smoking a bong, messing with the skaters, along with Todd Bridges, who also works there. They go to a convenience store and drink beer without paying and get chased by the clerk with a shotgun, though he joins them in creating chaos as well, including stealing a little girl's bike to teach Todd how to ride, even though he hits a car and interrupts an old couple having sex.
They return home, and Donny tells Todd he loves him, but Todd doesn't appear to feel that way about him yet. Ice passes out in the front lawn as they go inside. Donny ends up having sex with Steve's mom while Todd sees Jamie's wedding dress, thinking it's her, and starts humping it. He wakes up to Jamie screaming about the dress covered in puke, along with semen (which Jamie discovers after licking it). She yells at Todd and tells him to send the dress to the dry cleaners and to take off the earring. Ice then comes in after hearing the commotion, then goes back to banging Steve's mom.
Donny makes Jamie's family and the two Chinese servants breakfast when he gets a call from Todd. He tells him that he left Donny a present and is going to see his mom in prison. Donny rushes to stop him, but Ms. McGarricle (now Susan Sarandon) is already coming to talk to Todd and Donny. She is still in love with Donny, and he still wants her back. That's when Randall and his crew come in and tape the reunion, which angers Todd as he realizes what's really going on. He leaves, but not before hitting Donny and getting tasered, and he craps his pants. Because Todd didn't sign the release form, Donny will not get paid.
Donny goes home and hears Jamie on the phone with Steve, learning that she is cheating. He goes to the rehearsal dinner at night and tries telling Todd, but she manages to make up a cover story. He leaves again, shamed. As Jamie and Chad leave, Kenny (Nick Swardson), a very creepy guy who goes to the strip joint, tells Donny to go after Jamie and try and win her forgiveness. He gets into the hotel room after the clerks recognize him as the kid who banged his teacher. He goes up with two ice cream cones when he hears Jamie having sex. After sneaking in through the window, he discovers that Jamie is having sex with none other than...Chad! Donny is understandably grossed out, telling Chad he is a disgrace as a marine, until Jamie says that he only buys those outfits so their dad won't find out he's a jazz dancer (which Donny thinks is worse than the incest). Donny starts leaving to tell Todd, but Jamie says she will give him money to keep his mouth shut.
Donny calls Jim and tells him that he got the money, but he opens Todd's gift and discovers it's an oven mitt puppet from his childhood. This inspires Donny to move on and stop the wedding. He rounds up Ice, and they get to the wedding in time. That's when Donny and Todd reveal that they are father and son, and Donny rips Jamie's check, saying he'd rather go to prison than take her money. He forces her to tell Todd about her affair with Chad through a whisper, but Todd is so grossed out that he blurts it out to everybody, and Donny reveals she is also sleeping with Steve. Todd then quits his job and starts walking out, proudly accepting his name as Han Solo Berger, also telling Steve how Donny and Ice banged his mom. Jamie grabs a cake knife and starts running toward Todd, but Donny knocks her out with a bottle. Chad tries to attack, but he is knocked out by Father McNally. Donny, Todd, and Ice leave together.
Sometime later, Donny and Todd are having a little going-to-jail party at the strip club. Todd is now with Brie, and Donny accepts that he'll be going to jail for 3 years, because by then, Ms. McGarricle will be done with her sentence, and they can get together again. Just then, Jim comes in and tells Donny that the fat guy they bet on is running the marathon. Since Donny placed the bet, the money would come to $160,000. The patrons all cheer him on as the fat guy wins the marathon, and therefore, Donny will now have enough to avoid prison.
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