Elsa, princess of Arendelle, possesses the ability to create ice and snow. One night while playing, she accidentally injures her younger sister Anna. The king and queen seek help from trolls, who heal Anna and remove her memories of her sister's magic. The royal couple decides to lock the family away in their castle until Elsa learns to control her powers. Afraid of hurting her sister again, Elsa spends most of her time alone in her room, causing a rift between the sisters as they grow up. When the girls are teenagers, their parents are lost at sea during a storm.
When Elsa comes of age, the kingdom prepares for her coronation. Among the guests is the Duke of Weselton, a tradesman seeking to exploit Arendelle for profit. Excited to be allowed out of the castle again, Anna explores the town and meets Prince Hans of the Southern Isles, and the two immediately develop a mutual attraction. Despite Elsa's fear, her coronation goes off without incident. During the reception, Hans proposes and Anna hastily accepts. However, Elsa refuses to grant her blessing and forbids their marriage. The two sisters argue, culminating in Elsa's abilities being exposed to everyone as she is unable to control her emotions.
Panicking, Elsa flees the palace, inadvertently unleashing eternal winter on the kingdom in the process. High in the mountains far from Arendelle, she casts off restraint, building herself a solitary ice palace, and unknowingly brings to life her and Anna's childhood snowman, Olaf. Meanwhile, Anna sets out in search of her sister, determined to return her to Arendelle, end the winter, and mend their relationship. While getting supplies, she meets mountain man Kristoff and his reindeer Sven. She convinces him to guide her up the North Mountain. The group then encounters Olaf, who leads them to Elsa's hideaway.
Anna and Elsa are reunited, but Elsa still fears hurting her sister. When Anna persists in persuading her older sister to return, Elsa becomes agitated, and accidentally strikes Anna in the heart with her powers. She then creates a giant snow creature to run the friends out of her castle. As they flee, Kristoff notices that Anna's hair is turning white, and seeks help from his adoptive family of trolls. They are told that Anna's heart has been frozen, and unless it's thawed by "an act of true love," she will become frozen solid forever. Believing that only Hans can save her, Kristoff races back with her to Arendelle.
Meanwhile, Hans, who had gone on a search for Anna, encounters Elsa's palace. In the ensuing battle against the Duke's men, she is knocked unconscious and imprisoned back at the kingdom. There, Hans pleads with her to undo the winter, but Elsa confesses she doesn't know how. When Anna is reunited with Hans and begs him to kiss her to break the curse, Hans refuses and reveals that his true intention in marrying her is to seize control of Arendelle's throne. Leaving Anna to die, Hans charges Elsa with treason for her younger sister's apparent death.
Elsa escapes and heads out into the blizzard on the fjord. Olaf finds Anna and reveals Kristoff is in love with her. The two then travel rush onto the fjord to find him. Hans confronts Elsa and tells her Anna is dead because of her. In Elsa's despair, the storm suddenly stops, giving Kristoff and Anna the chance to reach each other. However Anna, seeing that Hans is about to kill Elsa, decides to throw herself between the two and subsequently freezes solid, blocking the blow.
As Elsa grieves for her sister, Anna begins to thaw; her decision to sacrifice herself to save her sister constitutes "an act of true love." Realizing love is the key to controlling her powers, Elsa is able to thaw the kingdom and even helps Olaf survive in summer. Hans is sent back to the Southern Isles to face punishment for his crimes against the royal family of Arendelle, and Elsa cuts off trade with Weselton. Anna and Kristoff share a kiss, and the two sisters reconcile, with Elsa promising never to shut the castle gates again.
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