Lemuel Gulliver (Jack Black) works a dead-end job in the mail room of a big company. He is introduced to Dan (T.J. Miller), a newcomer to the company who will be working under him. However, Dan is quickly promoted and becomes Gulliver’s boss. Depressed that he may never go anywhere in life, Gulliver ends up talking to journalist Darcy Silverman (Amanda Peet), who he has a crush on. He lies to her and says his dream is to become a writer and is able to convince her to let him write a report on his extensive travels around the world, all of which is a lie. Gulliver is not able to come up with anything interesting to write and so he copies and steals from other articles on the web. Darcy is impressed by his writing and presents Gulliver with an assignment. She wants him to travel to the Bermuda Triangle and write an article debunking all the legends and myths of ships mysteriously disappearing in the area being caused by extraterrestrials.
Once arriving in Bermuda, Gulliver rents out a boat and sails into the Bermuda Triangle. His boat is caught up in a freak storm and the boat capsizes and becomes overwhelmed by the huge waves. He washes up unconscious on the shore of Lilliput, where the population is microscopic. The tiny population claims him to be a beast and ties him to the ground. He is captured and imprisoned in a cave because they think he is too dangerous due to his large size.
In the cave, Gulliver meets another prisoner named Horatio (Jason Segel) who was put in prison by General Edward (Chris O'Dowd) because of his love for Princess Mary of Lilliput (Emily Blunt). General Edward also has a thing for her and wants to marry her and sees Horatio as competition for her affections. Soon, the island across from Lilliput begins an invasion. In the ensuing chaos, Gulliver breaks free from the cave and rescues Princess Mary from being kidnapped. He also manages to save her father King Benjamin (Billy Connolly) from a fire by peeing on it.
Gulliver is declared a hero by the Lilliputians. Gulliver makes up an exceeding number of lies to make himself look greater in the eyes of the Lilliputians. He claims to be the President of the United States, and that he is a living legend back home. Edward becomes jealous of the fame found by Gulliver due to the luxurious accommodations. He becomes further enraged when Gulliver is presented with the title of Honorary General of Lilliput.
The townspeople eventually finds the wreckage of Gulliver’s boat and his things including his cell phone. Gulliver finds that he has received a deluge of angry messages from Darcy. She has found out about his plagiarism. She now has to travel to Bermuda and take his place on the story since he has not returned any of her messages. She hates him now and no longer wishes to be friends. The next day, the island across from Lilliput again attacks the city because Edward has shut down the defenses around Lilliput as revenge against the people for putting Gulliver up on a pedestal. Gulliver, due to his gigantic stature, again defeats the other island since the cannonballs firing at him do nothing to him, though they do leave numerous welts all over his body.
Edward in a fit of rage and jealousy abandons Lilliput and joins the other side. Using one of Gulliver’s sci-fi magazines that they found on his boat, Edward builds a gigantic robot and pits it against Gulliver in battle. The robot defeats Gulliver, and he is forced to surrender. Embarassed and humiliated in battle, he is banished to the island across from Lilliput. Gulliver tells the Lilliputians the truth telling them he just a guy from the mail room and nothing more.
On the other island, Gulliver is captured by a young female giant and is forced to become her living doll playing tea time and other little girl games. Meanwhile, Darcy ends up imprisoned by the Lilliputians after being transported to Lilliput in the same way as Gulliver. Horatio goes over to the other island to rescue Gulliver and reveals to him that Darcy has been captured on Lilliput. Gulliver and Horatio just narrowly escape from the island after using a parachute from the girl’s dollhouse.
Arriving back on Lilliput, Edward again challenges Gulliver to a duel in order to free Darcy. Gulliver accepts and with the assistance of Horatio who is able to disable the robot’s cables, is able to defeat the robot. Horatio receives a hero’s welcome and King Benjamin gives permission to Horatio to court Princess Mary. Edward begins to grow even crazier after his defeat and threatens to kill Princess Mary. Princess Mary, sick of Edward, finally beats him up in frustration. Gulliver then makes peace with Lilliput’s rival island nations by quoting Edwin Starr’s “War”. Now that peace has returned to Lilliput, Gulliver and Darcy repair their boat and return to New York. They become a couple and receive success as fiction writers, writing about their experiences on Lilliput.
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