Mac (Seth Rogan) and Kelly Radner (Rose Byrne) are a young couple with a newborn daughter, Stella. The restrictions of parenthood make it difficult for them to maintain their old lifestyle, which alienates them from their friends Jimmy Blevins and his ex-wife, Paula. One day, the couple finds out that Delta Psi Beta, a fraternity known for their outrageous parties, has moved into an adjacent house. The fraternity's leaders, Teddy Sanders (zac Effron) and Pete Regazolli (Dave Franco), aspire to join Delta Psi's Hall of Fame by throwing a massive end-of-the-year party.One night, the couple ask Teddy to keep the noise down. Teddy agrees on the condition that Mac and Kelly always call him instead of calling the police. To earn Mac and Kelly's favor, Teddy invites them to join the party, which the couple agree to. At the party, Kelly meets Teddy's girlfriend, Brooke Shy, and Teddy shows Mac his bedroom, which includes a stash of fireworks and a breaker box that controls their power.The following night, Mac is unable to get in touch with Teddy to ask him to keep it down. Kelly convinces Mac to call the police, but Officer Watkins identifies them to Teddy. The following days, Delta Psi constantly hazes Mac and Kelly. The couple goes to the college dean, Carol Gladstone (Lisa Kudrow), and learn that the school has a three strikes policy; burning down their old house was Delta Psi's first strike.Mac damages the Delta Psi house by cutting down their water pipe, which floods the basement. The fraternity takes pictures of their dicks & sells it to raise $ 10,000 & make more than enough money to fix the damages.After failing to force the fraternity to move by damaging their house, Kelly manipulates Pete and Brooke into having sex and Mac gets Teddy to catch them. Teddy and Pete fight, which ends with a barbecue grill being rolled into the path of a passing car and injuring a professor, giving Delta Psi their second strike.To acquire evidence of Delta Psi's hazing, Kelly and Mac hire a pledge to stand up to Teddy to record him threatening retaliation. When Teddy instead shows him kindness, he reveals that Mac and Kelly hired him and also damaged their house. Teddy begins playing violent pranks on the couple.Mac & Kelly enlist the help of their divorced friend to get Teddy and Pete to get their last strike. Jimmy's ex-wife is being boned by one of the frat boys & Jimmy also wants revenge.Mac and Kelly send Teddy a counterfeit letter from Gladstone enabling them to have parties again (Gladstone had banned Teddy from having parties after the 2nd strike), and Teddy begins planning their end-of-the-year bash. Once the party is in full swing, the Radners call Watkins to complain about the noise. Teddy discovers the random strangers sent by Mac, Kelly and Jimmy. After finding a flyer about the party (Teddy did not circulate any fliers, & in fact it was Mac & Kelly advertising the party to catch Teddy in the act) and determining the letter is counterfeit, he stops the party just as Watkins arrives. Jimmy throws himself from the balcony to distract Teddy, allowing Mac and Kelly to sneak into Teddy's bedroom and restart the party using the breaker box. Teddy catches them and fights Mac, while Kelly lights one of the fireworks and shoots it at Watkins's patrol car. Teddy takes the blame for the party and convinces Pete to take the others and flee. Gladstone shuts the house down and Mac and Kelly return home, adjusting to their new lives.Four months later, Mac is at an outdoor shopping mall when he runs into Teddy, who is working as a shirtless greeter at Abercrombie & Fitch. The two greet each other warmly and Teddy tells Mac that he is attending night classes to complete his degree. Mac takes off his shirt and jokingly acts as a greeter with Teddy.Mac and Kelly later take pictures of Stella dressed in various costumes for a calendar. They get a call from Jimmy and Paula, who are attending a wild party and invite the couple to come, including Stella. Mac and Kelly decline, accepting their new roles as parents.