The film starts with Mike Stangle (Adam Devine) trying to sell liquor to a bar owner named Randy (Marc Maron). Mike uses his brother Dave (Zac Efron) to help him sell the stuff by pretending to be a stranger looking for good booze. Randy is used to Mike and Dave pulling this stunt, but he decides to buy three cases of their stuff anyway.
Mike and Dave are party animal man-children. Their parents Burt (Stephen Root) and Rosie (Stephanie Faracy) call them together to meet with them and their sister Jeanie (Stephanie Beard) and her fiancee Eric (Sam Richardson). Burt presents the boys with a slideshow of all the times they ruined family events with their shenanigans, including setting off fireworks dangerously and causing a girl to injure herself after she fell off a roof. Burt demands that the two of them bring respectable dates to Jeanie's upcoming wedding in Hawaii. Mike and Dave don't like the idea, but they agree to do it for Jeanie. We later find out that Mike is considered to be the black sheep of the family, and Dave is just roped into his foolishness.
We meet best friend Alice (Anna Kendrick) and Tatiana (Aubrey Plaza) as they stiff a cab driver for a ride. They go to their waitress job at a bar. Alice argues with her boyfriend Ronnie (Jake Johnson), forcing Tatiana to intervene. Alice then gets drunk and hops on a table to dance, causing both of them to get fired. The two of them go home. We learn that Alice got jilted at her own wedding, and she hasn't gotten over it. Tatiana proposes that they stop moping and seek out an adventure.
Mike and Dave place an ad to get dates, which gets tons and tons of replies due to the promise of a free trip to Hawaii. The brothers interview a bunch of girls, but none of them qualify as being respectable enough for them to take (and one of them is just a creepy dude in drag). They later go on "The Wendy Williams Show", where Alice and Tatiana see them talk about their trip. Thinking this is the adventure they need, they decide to clean up and make their moves.
The ladies catch the brothers leaving a restaurant one evening. Tatiana gets their attention by jumping in front of a moving car. Alice calls for help, leading Mike to jump in and try performing CPR on Tatiana. After "saving" her, the two pairs go out for the night and get to know each other. Alice says she manages a hedge fund while Tatiana makes up that she's a schoolteacher. Mike and Dave think they've found the perfect girls. They ask them if they want to go to Hawaii, and they accept.
Everyone heads on over to Hawaii. Alice and Tatiana manage to charm the pants off the whole family. Mike catches his cousin Terry (Alice Wetterlund), with whom he's always had a bitter rivalry, hitting on Tatiana. The ladies manage to convince Jeanie and Eric to go on an ATV ride in the spot where they filmed "Jurassic Park", even though the brothers planned to go swimming with dolphins.
The gang rides their ATVs to a hill. Alice and Tatiana ride off a slop and impress the guys with their moves. Mike is then goaded into riding down himself, despite Dave and Eric telling him not to. He goes for it, but he is unable to control the movement of the vehicle and he ends up hitting Jeanie in the face with one of the wheels, leaving a nasty bruise on her face.
Feeling partially responsible for what happened, Alice joins Jeanie at the spa. She and the maid of honor, Becky (Mary Holland), make Jeanie feel more tense, causing her to snap on all the ladies near her. Alice then talks to the masseuse (Kumail Nanjiani) and slips him some money to give Jeanie a truly relaxing experience. The masseuse gets naked and gives Jeanie such a great massage that she has an orgasm.
Mike walks in on this and screams. He then runs into the sauna where Tatiana is fingering Terry in exchange for Rihanna tickets. Mike confronts Tatiana, where she unintentionally admits that she and Alice saw their appearance on Wendy Williams and decided to score the free trip to Hawaii. Horrified, Mike runs to find Dave.
Dave and Alice are taking a walk around the area. Dave says he wants to work on a graphic novel, showing Alice some of his artwork. Alice admits to him about what happened with her ex-fiancee. The two appear to have a spark going on. Mike shows up and calls Dave over, ruining the moment. He tells Dave what he has seen and learned regarding Alice and Tatiana. Dave doesn't believe it, though.
The family is set to gather for the rehearsal dinner. Jeanie thanks Alice for hooking her up with the massage, mentioning that she's been stressed about everything so far. Alice helps her loosen up with ecstasy pills, not knowing that Jeanie has never done E before. They take their clothes off and set some horses free from their stable. Dave finds them and realizes Mike was telling the truth.
At the actual dinner, Mike tries to poison Terry by putting eye drops in her drink to make her sick as she goes onstage for a standup bit (he was inspired by "Wedding Crashers"). Dave stops him and argues with him backstage, not knowing that their mics got turned on. They reveal all the things they've seen during the trip and argue when Dave says Mike is the screw-up. Mike keeps trying to attack Dave, but he keeps getting punched hard in the face. When Eric hears about the massage, he confronts Jeanie, who's still tripping on E. She admits that she thinks Eric is kinda boring, leading to the two of them to call off the wedding.
Everyone starts preparing their bags to go home. Mike and Dave, plus Alice and Tatiana, all realize how badly they screwed up. The brothers cry like babies over ruining their sister's life while the ladies realize they have to care about other people's feelings. Both of them run to Jeanie and Eric's place to apologize. Their constant stupidity pushes Eric over the edge, causing him to flip out on all of them for ruining their weekend. Eric proves he's not boring as he got himself and Jeanie tickets to ride a hot air balloon around the world, even though Eric is afraid of heights. They reconcile and decide they still want to get married.
Mike, Dave, Alice, and Tatiana rush to get things ready for a last minute wedding. They manage to secure a venue and get the families gathered to watch Jeanie and Eric get married. Burt and Rosie are surprised that the boys managed to pull this off. After the wedding, Mike offers Tatiana the chance to work with him in selling his liquor since Dave is going to start his graphic novel. She agrees. Dave and Alice then become a couple and hook up.
At night, Mike and Dave perform a song for Jeanie. They then start to sing "This Is How We Do It" with help from Alice and Tatiana. They set off some fireworks, but unfortunately cause them to go off all around the party.
The film ends with Mike and Tatiana getting it on in the horse stables.
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