The movie begins by showcasing homemade videos of siblings Jack and Jill as they grow up. Through these clips, it becomes evident that Jack, played by Adam Sandler, is the more accomplished twin, while Jill frequently attempts to capture his attention through mischievous antics such as hurting him and the girls in their vicinity. Fast forward to adulthood, and we meet Jack, a prosperous advertising executive residing in Los Angeles, who enjoys a beautiful life with his wife, Katie Holmes, and their two children, Sofie and Gary.
However, Jack's serenity is disrupted by the impending holiday visit of his "identical" twin sister, Jill, also portrayed by Adam Sandler. Jack's frustration mounts as he is obliged to pick her up at the crack of dawn, at 4:00 am. The arrival is fraught with chaos, leading to multiple annoying incidents, rapidly testing Jack’s patience. Jill’s neediness and passive-aggressive behavior completely unsettle the tranquility Jack has cultivated.
As Thanksgiving dinner approaches, Jill’s antics spiral into chaos—she disrupts the gathering with loud comments, labels a guest as homeless, and darts into the woods with her pet cockatoo. Forced by his wife to mend fences, Jack visits Jill in the woods, where she reveals a list of aspirations she wishes to fulfill before her departure, including appearing on a game show, horseback riding, and touring a studio. To his dismay, Jill decides to extend her stay through Hanukkah.
Meanwhile, Jack’s work life further intensifies as he aims to land Al Pacino for a Dunkin’ Donuts commercial, a task that becomes more complicated with Jill in town. In her quest for love, Jill tries online dating without much luck—until the frustrated Jack impersonates her, leading to a flood of responses, but her date with "Funbucket," played by Norm Macdonald, is a comedic disaster when he flees in terror.
In a bid to alleviate his guilt, Jack invites Jill to a Lakers game, where Al Pacino is supposed to be. During this outing, Pacino unexpectedly becomes smitten with Jill, even managing to convey his phone number in a rather unconventional manner—written on a hot dog with mustard and ketchup. Jack secretly hopes that Jill will head back home by New Year's Eve since he and his family have planned a cruise.
As Jack's friends throw a combined birthday party for him and Jill—unaware of her existence—Jill becomes the center of attention, culminating in an amusing scenario. A twist occurs when Pacino takes Jill to his residence, where she joyfully selects a cake. Strangely, the next day, Jill nearly departs, but destiny intervenes when Jack’s gardener, Felipe, organizes a family picnic for her. As Jill forms a bond with Felipe’s kids, it becomes apparent she craves connection.
In an unorthodox arrangement, Pacino agrees to do the commercial if Jack can arrange a date with Jill. Faced with this challenge, Jack disguises himself as Jill to keep her from realizing the arrangements being made. A helicopter picks up “Jill” for an extravagant date at a castle, but as Jack witnesses Pacino’s deep feelings for his sister, he grapples with his affection for her as well and races back to the ship, only to discover that Jill has left for the Bronx, having perceived Jack’s true motives for wanting her to stay.
Reunited at a restaurant where they remember their late mother, Jack and his family celebrate with Jill. An encounter with their former classmate, Monica, adds chaos, but Jill stands up for Jack’s wife. The evening concludes at Jill’s house, which Felipe has romantically adorned, prompting a sweet moment between them. However, Pacino’s arrival—dressed in costume after his performance in Man of La Mancha—complicates matters as he makes his feelings known but ultimately steps back to respect Jill's newfound happiness.
The film wraps up with a vibrant commercial featuring Pacino promoting a Dunkaccino alongside a catchy rap song. Despite the potential of the ad, Pacino expresses his disdain, demanding Jack destroys all copies and prevents anyone from viewing it.
