Elsa, princess of Arendelle, possesses cryokinetic powers, with which she is able to produce ice, frost, and snow at will. One night while playing, she accidentally injures her younger sister, Anna. The king and queen seek help from the troll king, who heals Anna and removes her memories of Elsa's magic. The royal couple isolates the children in their castle until Elsa learns to control her powers. Afraid of hurting Anna again, Elsa spends most of her time alone in her room, causing a rift between the girls as they grow up. When the girls are teenagers, their parents die at sea during a storm.
When Elsa comes of age, the kingdom prepares for her coronation. Among the guests is the Duke of Weselton, who seeks 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 quickly 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 sudden marriage. The sisters argue, culminating in the exposure of Elsa's abilities during an emotional outburst.
Panicking, Elsa flees the castle, while inadvertently unleashing an eternal winter on the kingdom. High in the nearby mountains, she casts off restraint, building herself a solitary ice palace, and unknowingly brings her and Anna's childhood snowman, Olaf, to life. Meanwhile, Anna sets out in search of her sister, determined to return her to Arendelle, end the winter, and mend their relationship. When obtaining supplies, she meets an iceman named Kristoff and his reindeer, Sven, and convinces Kristoff to guide her up the North Mountain. On their journey, the group encounter Olaf, who leads them to Elsa's hideaway.
Anna and Elsa reunite, but Elsa still fears hurting her sister. When Anna insists that Elsa return, Elsa becomes agitated and her powers lash out, accidentally striking Anna in the heart. Horrified, Elsa creates a giant snow creature to drive Anna, Kristoff, and Olaf away from her palace. As they flee, Kristoff notices Anna's hair turning white, and deduces that something is very wrong. He seeks help from the trolls, his adoptive family, who explain that Anna's heart has been frozen by Elsa. Unless it is thawed by an "act of true love", she will become frozen solid forever. Believing that only Hans can save her with a true love's kiss, Kristoff races back with her to Arendelle.
Meanwhile, Hans, leading a search for Anna, reaches Elsa's palace. In the ensuing battle against the Duke's men, Elsa is knocked unconscious and imprisoned in Arendelle. There, Hans pleads with her to undo the winter, but Elsa confesses that she does not know how. When Anna reunites 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, he 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; they then escape onto the fjord to find him. Hans confronts Elsa, telling her Anna is dead because of her. In Elsa's despair, the storm suddenly ceases, giving Kristoff and Anna the chance to find each other. However, Anna, seeing that Hans is about to kill Elsa, throws herself between the two just as she freezes solid, blocking Hans' attack.
As Elsa grieves for her sister, Anna begins to thaw. Realizing love is the key to controlling her powers, Elsa thaws the kingdom and helps Olaf survive in summer. Hans is deported back to the Southern Isles to face punishment for his crimes against the royal family of Arendelle, while Elsa cuts off trade with Weselton. Anna and Kristoff share a kiss, and the two sisters reconcile. Elsa promises never to shut the castle gates again.
