The movie kicks off with Mike Stangle (Adam DeVine) attempting to sell some liquor to a bar owner named Randy (played by Marc Maron). To help him with this, Mike enlists his brother Dave (Zac Efron), who pretends to be a stranger on the lookout for quality booze. Despite Randy's familiarity with the brothers' antics, he surprisingly decides to purchase three cases of their product.
Both Mike and Dave are portrayed as immature party enthusiasts, often causing trouble. Their parents, Burt (Stephen Root) and Rosie (Stephanie Faracy), gather everyone, including their sister Jeanie (Stephanie Beard) and her fiancé Eric (Sam Richardson), for a family meeting. Burt showcases a slideshow marking all the occasions when Mike and Dave wreaked havoc on family events—like dangerously setting off fireworks and causing an injury to a girl who fell off a roof. Burt insists that the brothers must bring respectable dates to Jeanie's upcoming wedding in Hawaii. Reluctantly, they agree for Jeanie's sake, although Mike is seen as the family's black sheep while Dave is dragged along for the ride.
As we delve into the story, we meet best friends Alice (Anna Kendrick) and Tatiana (Aubrey Plaza), who are portrayed as rebellious women. They dodge a cab fare on their way to their waitress job at a bar, leading to a drunken night where Alice ends up dancing on tables. This behavior leads to their termination from the bar, and we discover Alice still grapples with the heartbreak of her own wedding being called off. Tatiana suggests they embark on a daring adventure to lift their spirits.
In a bid to find dates, Mike and Dave post an ad that garners an overwhelming response due to the allure of a free trip to Hawaii. After interviewing numerous candidates, they fail to find anyone suitable, encountering ridiculous applicants, including one bizarre individual in drag. On "The Wendy Williams Show," Alice and Tatiana, intrigued by the brothers' offer to travel, decide to spruce themselves up and seize the opportunity for an adventure.
The two ladies make a splash by nearly causing an accident to catch Mike and Dave's attention, prompting Mike to jump in and attempt CPR on Tatiana after her stunt. Following a wild night out, the brothers feel they have found their dream dates when Alice, posing as a hedge fund manager, and Tatiana, claiming to be a schoolteacher, agree to accompany them to Hawaii.
Once in Hawaii, the brothers introduce Alice and Tatiana to the family, who are charmed by the women, especially when the girls persuade Jeanie and Eric to join them on an ATV excursion, steering away from the agreed-upon dolphin swimming expedition. During an exhilarating ride, Mike, goaded into a challenge, accidentally collides with Jeanie, resulting in a terrible bruise on her face.
Feeling guilty, Alice tries to assist Jeanie by accompanying her to the spa. However, their antics backfire as they inadvertently add to Jeanie's stress, leading to a hilarious but embarrassing situation when Jeanie enjoys a surprisingly intimate massage. Mike's shocking discovery of this leads to further chaos as he confronts Tatiana and learns the truth about their initial intentions.
As the story unfolds, Dave and Alice share a moment that hints at a budding romance, which Mike interrupts with news of their real relationship status. During the rehearsal dinner, further complications arise when Alice gives Jeanie ecstasy, resulting in wild antics that put the wedding in jeopardy. Amidst family revelations and confrontations, Mike's attempt to sabotage Terry's stand-up performance leads to surprising declarations, and Eric and Jeanie find their relationship on thin ice.
The brothers and their companions realize the gravity of their blunders and rush to make amends. Eric confronts Jeanie about her feelings of boredom, prompting them to address their issues and reaffirm their love. As the brothers scramble to put together a last-minute wedding, Burt and Rosie find themselves astonished at what the boys can accomplish.
After a whirlwind of a wedding, Mike proposes a business partnership to Tatiana while Dave and Alice solidify their relationship. The night culminates with the brothers performing a song for the newlyweds, leading to the chaos of fireworks that spirals out of control. The film concludes on a comedic note as Mike and Tatiana sneak away for a private rendezvous in the stables, leaving behind the laughter of the celebrations.
