The wizard Vitruvius attempts to protect the "Kragle", a superweapon, from the evil Lord Business. He fails to do so, but warns Lord Business of a prophecy where a person called the "Special" will find the Piece of Resistance capable of stopping the Kragle.
8 and a half years later, Emmet Joe Brickowski, an ordinary construction worker with no special qualities, comes across a woman, Wyldstyle, who is searching for something after hours at Emmet's construction site. When he investigates, Emmet falls into a hole and finds the Piece of Resistance. Compelled to touch it, Emmet experiences vivid visions and passes out. He awakens elsewhere, with the Piece of Resistance attached to his back, in the custody of Bad Cop, Lord Business' lieutenant (whose head sometimes turns around to reveal his other side, Good Cop). There, Emmet learns Business' plans to destroy the world with the Kragle. Wyldstyle rescues Emmet and takes him to Vitruvius, who explains that he and Wyldstyle are "Master Builders" capable of building anything they need, both with great speed and without instruction manuals. Years ago, Lord Business rose to power, his disapproval of such anarchic creativity resulting in him capturing many of them. As the "Special", Emmet is destined to defeat him, yet Wyldstyle and Vitruvius are disappointed to find Emmet displays no creativity.
Lord Business plans to use the Kragle (a tube of Krazy Glue with some of the logo's letters rubbed out) to freeze the universe perfectly in place. Bad Cop tracks down Emmet and Wyldstyle, who are rescued by her boyfriend, Batman. He takes them to a meeting of the remaining Master Builders. Unimpressed with Emmet, they refuse to fight Lord Business. Bad Cop and his forces attack and capture all the Master Builders except for Emmet and a few others. Emmet believes the Master Builders' weakness is that their individual creativity prevents them from working together. He devises a team plan to infiltrate Lord Business' headquarters. As Emmet and his allies are captured and imprisoned, Vitruvius attempts to fight back but is killed; with his dying words he admits the prophecy was made up. Business throws the Piece of Resistance off the edge of the universe, sets his headquarters to self-destruct, and leaves with the Kragle while leaving Bad Cop behind. Vitruvius' ghost tells Emmet that even if the prophecy isn't real, Emmet can still save the world. Emmet, tied to the self-destruct mechanism's battery, sacrifices himself for his friends, flinging himself off the edge of the universe.
Inspired, the Master Builders escape and rally with the help of Bad Cop. Soon, Lego people across the universe are building their own creative weapons. The Master Builders lead the charge against Business.
Emmet finds himself in the real world, where the events of the story are being played out within the imagination of a boy, Finn. His father "The Man Upstairs" chastises his son for ruining his father's Lego set by mixing characters with the wrong playsets, and originating hodgepodge creations. Finn argues that Lego are for children, but his father prefers to Krazy Glue his perceived perfect creations together permanently, as this is how adults play with Lego. In the Lego world, Lord Business' forces gain the upper hand. Realizing the father will glue all the Lego in place, Emmet wills himself to move and falls off the table, gaining Finn's attention. Finn returns Emmet to the Lego set, where Emmet builds a massive robot to assist his friends before confronting Lord Business. In the real world, Finn's father looks at his son's creations again and finds himself impressed. Realizing his son based the evil Lord Business on him, the father has a change of heart and allows his son to play with his Lego however he sees fit. In the Lego world, Emmet convinces Lord Business that Business, too, is special, as is everyone. Moved by Emmet's speech, Business destroys the Kragle and unfreezes his victims.
With the world saved, Emmet celebrates with his friends, and Wyldstyle, whose real name is Lucy, becomes his girlfriend. However, alien Duplo beings beam down, announcing their intentions to invade, due to the father allowing Finn's little sister to play with his Lego set as well. Then at the end they get captured by the Duplo toys