Princess Elsa was born with the power to create ice and snow, a power she's kept secret most of her life, even from her younger sister Princess Anna, and for good reason: one night, when the girls were little, Elsa accidentally struck Anna in the head with a blast of icy magic, which hurt Anna badly and turned a streak of her hair platinum-blonde.
Since then Elsa has worn gloves to conceal her magic, and kept herself in semi-isolation to avoid another accident, shutting out poor Anna, who had no memory of the incident. Even after the deaths at sea of their parents, the King and Queen of Arendelle, the sisters rarely interacted.
Now that Elsa has come of age at 21, she is about to be crowned Queen. Many dignitaries are invited to the momentous event, including the greedy duke of Weselton and Prince Hans of the Southern Isles.
On the evening of the coronation, Elsa's sister Anna falls in love with the charming Prince Hans, and the two become engaged. Anna asks for Elsa's blessing, but Elsa refuses to give it, and the sisters begin to argue rather heatedly.
In an emotional outburst, Elsa accidentally unleashes her powers before the whole court and flees from the kingdom in a panic, inadvertently trapping it in eternal winter as she does so. Anna sets off to find her to make things right again, leaving Hans in charge of the kingdom.
Elsa eventually finds herself on a high precipice on a mountain and, deciding to live there in isolation, where she can't hurt anyone, builds an elaborate palace of ice.
Anna, on her side, comes across mountain man Kristoff and his reindeer Sven and a talking snowman named Olaf, who was created by Elsa, and together they travel on in search of the newly crowned Queen.
At last they arrive at the ice palace, where Anna tells Elsa of the wintry curse on Arendelle. Horrified, Elsa confesses she doesn't know how to undo her magic and break the curse, and as the sisters begin once more to argue, Elsa loses control of her emotions, making her powers manifest again, and accidently strikes Anna in the heart.
Kristoff takes the injured Anna to his adoptive family, the rock trolls, to save her. Grand Pabbie, the leader of the trolls, reveals that Anna's heart has begun to freeze, and only an "act of true love" can melt it and reverse the spell before she freezes solid.
Thinking her fiance, Prince Hans, might be able to save her, Kristoff rushes Anna back to Arendelle.
Meanwhile, Hans and the duke's men, who had gone looking for Anna, find Elsa's ice palace and capture her after she's knocked unconscious in a violent struggle.
Once back in Arendelle, Hans reveals to Anna his true goal to seize the throne by letting her die and killing her sister.
Elsa awakens to find herself in a dungeon cell in Arendelle, but manages to escape her confinement by using her powers, while Olaf finds Anna and learns of Han's betrayal.
When a heartbroken Anna admits she doesn't even know what love is, Olaf tells her it is "putting someone else's needs before yours", using Kristoff as an example. Seeing Kristoff is near and realising he loves her, Anna resolves to get to him in time to break her curse.
Hans soon catches up to Elsa and attempts to kill her with a swing of his sword, but Anna jumps in the way and, freezing solid, stops him and breaks the sword. Devastated, Elsa breaks down and cries over Anna's frozen body. But it isn't long before Anna begins to thaw out, the act of true love being the sacrifice she made to save her sister.
Realising that love is the key to controlling her magic, Elsa is able to reverse the spell on Arendelle. The sisters are finally reunited and reconciled; Elsa is welcomed back as Queen; Hans is arrested; and Anna and Kristoff share their first kiss.