Staying at the Penthouse Suites in the Hilton Hawaii resort, Danny (Adam Sandler) and Palmer (Brooklyn Decker) don't spend as much time as they'd hope together - seeing as they are joined by Katherine (Jennifer Aniston), playing the role of the fake ex-wife, her kids and Danny's cousin Eddie (Nick Swardson) as the German sheep farmer 'Dolph Lundgren', meant to be 'Devlin the ex-wife'/Katherine's new boyfriend.
When Katherine has just about had enough of it and is about to leave, she bumps into her arch rival and "fr-enemy" - the real Devlin (Nicole Kidman). Devlin is still as obnoxious as ever, and is married to a city guy who claims to have invented the iPod. Katherine herself spins a web of lies, claiming that she is married to Danny, before she quickly makes an exit.
The next day Danny spends some quality time with the kids - even teaching young 'Bart' how to swim. Palmer likes him even more when the women return from a spa day. Devlin shows up at the poolside ... to save the day Eddie pushes Palmer into the water, and keeps her head under the water! Katherine has no choice but to accept Katherine's request that the two couples meet later for dinner.
That night Danny and Katherine leave the kids with a babysitter, while Eddie takes Palmer far away to a local restaurant, surrounded by farm animals. Danny and Katherine grow closer, and finally open up to reveal their true feelings about each other. Meanwhile, at the other restaurant, Eddie continues the backstory about his character when suddenly - a sheep outside is choking! The locals call for help - and Palmer tells them they are in luck as 'Dolph' is an online sheep expert! He gets outside to try and revive the sheep. After trying mouth-to-mouth ("Sheep-P-R" as he calls it) and a few elbow drops, he performs the Heimlich Maneuver, and the sheep is breathing again.
At the upmarket restaurant, Katherine manages to get into a Hula contest with Devlin - and wins this time. Walking back to her suite with Danny, they almost kiss when they are disturbed by the kids.
Danny wants to see her again - but bumps into Palmer and Eddie in the hotel lobby -- closely followed by Katherine. Eddie has spilt the beans about the ring - and Palmer accepts a proposal that wasn't really made. Katherine is devastated, but hides her feelings.
That night no one can sleep. Danny phones Katherine for advice, but she tells him to make up his own mind - and she'll be leaving for a new job in New York.
The next day, Katherine is drinking away her sorrows at an outside bar, when she is joined by Devlin. She comes clean about her life. Devlin can't believe that they weren't together, and admits that her own husband is gay, and they are seeking a divorce. And he didn't invent the iPod but got his money when he was hit by a baseball. Danny overhears Katherine say that she loves him.
He tells Katherine that he didn't get married that morning - as the only person for him was not at the ceremony. They kiss.
Palmer leaves, but on the flight home meets a tennis pro her age (Andy Roddick), who also shares her love for N-Sync.
Later, Danny and Katherine marry, and she's a big hit with his family. He doesn't need to pretend to be married anymore.