The film opens up with several sets of twins talking about their experiences growing up. We then see footage of Jack and Jill (Adam Sandler) growing up, with Jack getting more attention than Jill.
We switch to modern day where Jack is on the set of a commercial with Regis Philbin trying to sell Pepto-bismol. Regis complains to Jack that the muppet who is co-starring in the commercial has funnier lines than him. Jack's co-worker then comes to Jack and says that Dunkin Donunts, one of their biggest clients, is threatening to pull out from their firm if they do not get Al Pacino to do their next commercial. Jack's assistant then tells Jack that his sister, Jill, will be arriving at the airport early in the morning. We see Jack wake up to his wife (played by Katie Holmes) who tells him to think good thoughts about Jill while picking her up. Jack goes to the airport where Jill is the only person in airport luggage area. In the car, Jack and Jill bicker on the way to Jack's house.
At dinner the next night for Thanksgiving, Jill comes into the dining room and is upset that she is not able to sit next to Jack. Jack's daughter asks Jill if Jack and Jill have ESP, Jill states they do while Jack insultingly denies it. Jill, upset, runs out of the room and falls into Jack's large backyard garden. Jack's wife comforts Jill. The next day Jill gives Jack a list of all the things she wants to do while she's in Californina visiting Jack and his family. Jack tries to get her to tell him when she's leaving, but Jill eventually tells Jack that she has an open ended ticket and can leave whenever she wants, and she suggests that she stay until the end of Hanukkah. We then see Jill doing the things she wanted to do on her list: go to a costume store, ride a pony and then hang out with Drew Carey on Wheel of Fortune.
At the movies, Jack's son realizes that Jack and Jill are more similar than Jack would like to admit. Jack and his wife are talking about Jill and the next morning the whole family talks about how Jill likes men, but men do not seem to like her back. Jack begins to realize that Jill may feel lonely. The whole family decides to get Jill on a dating site, even though Jill does not know how to use a computer. The family sets up Jill's account, and she does not receive any hits. Jack, later that night, goes back to Jill's account and exaggerates her account file, the next morning Jill has over 100 hits. On a date with a guy from the dating site, Jill is upset when her date acts funny, mainly because she is not the hot babe described on her account page. When Jill gets home, the family press her for details on her date, and she runs to her room and cries. Jack invites Jill to a Lakers games where they run into Johnny Depp and Al Pacino in disguise. Pacino falls for Jill instantly, much to Jill's discomfort.
At a party for Jack and Jill's birthday, Al Pacino shows up and takes Jill to his house and his private chef. The next morning, Jill runs into the gardener Felipe. Felipe invites Jill to a family gathering where Jill enjoys herself. Pacino comes over and asks Jack where Jill is. Al lays on Jill's bed while Jack watches. Al believes that Jack is hiding Jill. When Jack tells Al that Jill is leaving, Al tells Jack that he'll do the commercial if Jack sets Al up with Jill. After Al leaves, Felipe drives Jill back to Jack's house, and Jill, who is having a bad reaction to chimichangas, runs out of Felipe's car and into the bathroom. Jack, who is now trying to be nice to Jill, invites Jill to go on the family cruise for the holiday, while Jill is in the bathroom.
Jack calls Al, who is starring in a Shakespearean play. Al interrupts his scene and talks in front of the guests. Al starts quoting The Godfather and the crowd erupts in cheers. Al wants to speak to Jill and Jack (who imitates Jill's voice) talks to Al and agrees to meet up with him. Jack tries to get Jill to go out with Al, who refuses. Jack then decides to dress up as Jill and go out on the date with Al himself dressed as Jill. Al has a helicopter hover over the cruise ship, and Jack (as Jill) gets flown over to a castle. On their date, Al tells Jack (as Jill) that he likes Jill because he sees that Jill has always been second best to Jack and that she deserves happiness. Jack begins to see the errors of his way and runs out of his date with Al.
Jack tries to get in touch with Jill who has left the cruise-ship and has decided to go to a New Years Eve party where old high school classmates attend. Jill's nemesis Monica (played by David Spade as a women) taunts Jill. Jill tells her former classmates that she was with Al and got tired of the Hollywood lifestyle. Jack surprises Jill and sticks up for his sister. In their twin language, Jack tells Jill that he loves her and misses her. Monica starts a fight with Jack's wife. Jill then fights Monica and Pacino pops up in character for a film that he will be working on. Jill lets him down and tells him that he's not interested. Jack drives Jill down a lane with decorated Christmas houses and Jill sees Felipe and his kids. Felipe tells Jill that he loves her and wants to be with her. Jill happily accepts by jumping on him.
We then see the commercial of Al Pacino for Dunkin Donuts who sings that he would now like to be called Dunkin Chino. After the commercial in the editing room with Jack, Jack tells Al he needs to lose two seconds, and Al replies Jack needs to burn the whole commercial because it cannot be seen.
The film ends where it began, with sets of twins talking about their experiences growing up with each other.
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