This soulful, family-oriented comedy opens with homemade videos of fraternal twins Jack and Jill Sadelstein growing up. As the videos progress, it seems that Jack is the more gifted twin, with Jill constantly trying to get his attention by hitting him, hurting girls around him, etc.
The story shifts to the present, where an adult Jack (Adam Sandler) is a successful advertising executive in Los Angeles with a beautiful wife, Erin (Katie Holmes), and two kids, Sofie (Elodie Tougne) and Gary (Rohan Chand), while Jill (Sandler) never left the working-class neighborhood they grew up in. Jill recently inherited her childhood home, having lived with their mother until her death one year ago.
Jack is incredibly irritated about the Thanksgiving visit of his "identical" twin sister Jill, as he has to pick her up at 4:00 a.m.. Jack's patience is soon stretched increasingly thin. Jill ruins Thanksgiving dinner by talking loudly, calling one guest homeless, then running away into the woods with her pet cockatoo. Jill has a list of things she wants to do—be on a game show, go horseback riding, and do a studio tour. She has an open-ended plane ticket and decides to stay until the end of Hanukkah.
Jack's agency client, meanwhile, wants him to somehow get actor Al Pacino to appear in a Dunkin' Donuts commercial. Jack isn't sure how he's supposed to make that happen. Jill tries online dating, but doesn't get much of a response until Jack poses as Jill and alters her profile, leading to more than 100 responses.
When her date, "Funbucket" (Norm Macdonald), meets her, however, he hides in the restaurant bathroom. Feeling guilty, Jack takes her to a Lakers game where Pacino is supposed to be. Pacino pays little attention to Jack, but is surprisingly taken with Jill. He gives her his phone number on a hot dog, written in mustard and ketchup.
Jack is hoping Jill would leave by New Year's Eve, since the family is going on a cruise. Friends throw him a birthday party and extend it to Jill as well, having never known that he even had a sister. Pacino invites Jill to his home but she resists his advances and abruptly leaves.
The next day, Jack's Mexican gardener, Felipe (Eugenio Derbez), takes Jill to a family get-together to have fun. Jack doesn't want Jill to be alone on New Year's, so he invites her on the cruise, and she gladly accepts.
A proposition is made by Pacino that he will do the commercial if Jack gets him a date with Jill. Jack disguises himself as Jill and goes on the date in her place. Jill starts to suspect that the only reason Jack invited her on the cruise is to persuade Pacino to do the Dunkin' Donuts commercial.
When she phones Jack, he answers as Jill, and then she hears Pacino, confirming her suspicions. Jack realizes how much he loves his sister and races back to the ship, but learns that Jill has gone back home to The Bronx. At a restaurant on New Year's Eve, there with a picture of the twins' dead mother, she comes across a group of old classmates that used to make fun of her, led by Monica (David Spade).
They belittle her until Jack and his family show up. Jack and Jill speak in their made-up twin language, then Monica gets Erin in a headlock, but Jill saves her. Pacino also turns up at the party, dressed as the Man of La Mancha, to proclaim his love for Jill. She turns him down and goes home, where Felipe declares his love. She is overjoyed and embraces him.
The television commercial is made, with Pacino starring and singing as he promotes a new coffee -- "Dunkaccino"—with a rap song. But when Jack shows it to him, Pacino hates it and tells him to destroy every copy of this film and threatens death to all those who have seen it.