Deeply disheartened by his uninspiring job in the mailroom of a New York City newspaper, Lemuel Gulliver (Jack Black) decides to express his feelings to journalist Darcy Silverman (Amanda Peet). He manages to persuade her that he possesses a wealth of experience from "travels" around the world, fabricating tales about his aspirations to become a writer. As he grapples with writer's block and doubts that Darcy would want to associate with someone from the mailroom, he resorts to copying articles from various online publications.
The following day, impressed by his supposed talent, Darcy assigns Gulliver a task: to travel to the Bermuda Triangle and compose an article debunking the myth of ships mysteriously vanishing in the region due to alien activity. Upon reaching Bermuda, Gulliver rents a boat to journey into the triangle. However, after dozing off while steering the ship, he finds himself caught in a fierce storm and is ultimately swept away by a waterspout. He awakens, unconscious, on the shores of Lilliput, where the tiny inhabitants regard him as a "beast". Initially imprisoned due to his immense size, Gulliver meets fellow prisoner Horatio (Jason Segel), who has been incarcerated by General Edward (Chris O'Dowd) for his affection towards Princess Mary (Emily Blunt), whom Edward covets for himself.
As the island of Blefuscu launches an operation to kidnap Princess Mary, Gulliver manages to escape from his labor and rescues her just in time. He even saves her father, King Theodore (Billy Connolly), from a fire using an unconventional method – urinating on the flames. Lilliput’s citizens hail Gulliver as a hero, and he concocts a series of grand lies, claiming to be the President of the United States, with Yoda as his Vice-President and a living legend in his country.
Enraged by the lavish lifestyle and accolades bestowed upon Gulliver, Edward seeks revenge. Matters escalate when the townspeople discover Gulliver's boat and belongings, leading to a series of angry voicemail messages from Darcy, who informs him that she needs to take his place in Bermuda and is upset about his dishonesty.
The following day, chaos erupts as the Blefuscian Navy besieges Lilliput, and Edward disables the city's defense mechanisms out of spite. In an epic showdown, Gulliver manages to fend off the invading armada, albeit suffering from numerous welts. As Princess Mary distances herself from him, Edward betrays Lilliput, enlisting assistance from the Blefuscans and utilizing designs from Gulliver’s sci-fi magazines to build a robot.
When the Blefuscans invade Lilliput with the robot, Edward forces Gulliver to publicly acknowledge his true identity as "just the guy from the mail-room." Subsequently, Edward banishes Gulliver to the shores of Brobdingnag, where he is captured by Glumdalclitch, a giant girl who makes him her living doll. In an amusing twist, Horatio, heartbroken after being rejected by Mary, finds Gulliver and reveals that Darcy has also become a captive of the Blefuscians after getting lost in the Bermuda Triangle.
Through daring maneuvers, including escaping with a parachute scavenged from a downed U.S. pilot, Gulliver rejoins Horatio. They face Edward one final time, not only for the liberty of Lilliput but for its very survival, as Edward threatens destruction if he loses. With Horatio's help, they conquer the machine's deadly weaponry, leading Horatio to become the hero and receive the king's blessing to court Mary. In a surprising twist, the princess confronts Edward, physically overpowering him in a moment of rebellion.
Ultimately, Gulliver reconciles the tensions between the rival islands by invoking the spirit of peace through Edwin Starr's "War." Together with Darcy, he sets sail back to New York on the repaired boat, concluding their adventure hand-in-hand, leaving the audience with a sense of hope and renewal.
