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 sudden marriage. The sisters argue, culminating in Elsa's abilities being exposed in an emotional outburst.
Panicking, Elsa flees the palace, inadvertently unleashing an eternal winter on the kingdom in the process. High in the nearby mountains, 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 reunite, but Elsa still fears hurting her sister. When Anna persists in persuading her sister to return, Elsa becomes agitated and accidentally strikes Anna in the heart with her powers. Horrified, she creates a giant snow creature to run the friends away before she may hurt anyone again. As they flee, Kristoff notices Anna's hair is turning white, and deduces something is very wrong. He seeks help from his adoptive family of trolls, who explain that Anna's heart has been frozen. 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, leading a search for Anna, reaches Elsa's palace. In the ensuing battle against the Duke's men, Elsa 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 reunites with Hans and begs him to kiss her to break the curse, Hans refuses and reveals that his true intention in marrying her was 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. The two then 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 ceases, 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, since 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.