Minions are small, yellow creatures who have existed since the beginning of time, evolving from yellow single-celled organisms into beings which exist only to serve history's most despicable masters. After rolling their leader T-Rex down a volcano, getting their caveman leader eaten by a bear, crushing a Pharaoh under the pyramids while his people survive, burning Count Dracula on his birthday party, and accidentally firing a cannon at Napoleon, the Minions are driven into isolation and start a new life in a massive cave in the Arctic. After many years, the Minions become depressed, restless, and unmotivated without a master to serve. To regain their dignity and sense of purpose, Kevin, one fearless Minion, decides to set out to find a new master and asks for help. Stuart, a musically inclined Minion, and Bob, a young and inexperienced but enthusiastic Minion, are recruited.

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The trio journeys to New York, where the year is 1968. After spending the day attempting to blend in, the Minions end up in a department store for the night, where they discover a hidden commercial broadcast for villains advertising Villain-Con; a convention for villains and supervillains in Orlando. The trio hitchhikes a ride with the Nelson family and impress them with their accidental villainy. At the convention, they see Scarlet Overkill, the first female supervillain, who hires them and takes them to her home in the United Kingdom.

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Scarlet explains the plan to steal the Imperial State Crown from the Queen of the United Kingdom and promises to reward the Minions if they steal it but kill them if they do not. Her husband Herb supplies them with inventions to aid in the heist, but they are nearly caught when they break into the Tower of London, which leads to a chase that ends with Bob crashing into the Sword in the Stone and pulling it free, removing the Queen from the throne and becoming King Bob. Enraged, Scarlet confronts the Minions, so Bob abdicates the throne in her favor. Undeterred because she feels betrayed, Scarlet imprisons the three in a dungeon to be tortured by Herb before her coronation, but they escape with the intention to apologize to Scarlet.

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Making their way to Westminster Abbey, the three of them interrupt the coronation by accidentally dropping a chandelier on Scarlet, who orders their execution. Dozens of villains chase them and Bob and Stuart are caught, while Kevin finds his way into a pub. He sees Scarlet on a television, promising that she will kill Stuart and Bob if Kevin does not show up by dawn. Kevin sneaks into Scarlet's home to steal weapons but inadvertently triggers a machine Herb was building and enlarges himself. He tramples through London, rescuing his friends just as the other Minions reunite with them. Scarlet tries to eradicate them, but Kevin swallows a massive missile she fires at them. Scarlet and Herb attempt to escape with her rocket-dress, but Kevin holds onto it. The missile detonates, causing it to explode. After the Minions briefly mourn him, Kevin returns, having returned to his normal size.

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The original Queen from the earlier part of the story receives her place on the throne and crown back and rewards Bob with a tiny crown for his teddy bear and Stuart an electric guitar, and she knights Kevin for his heroism. She suddenly realizes that her crown is missing, and Kevin leads a chase after Scarlet and Herb, who survived the explosion and are fleeing through the crowd with the bejeweled crown until they are suddenly frozen in place by a young Gru, who steals the crown from them and takes off in a rocket-powered motorbike while the Minions stare in awe. Bob gives his crown to the frozen Scarlet, and the Minions see Gru as their new potential master and give chase to follow him home.