Eight-year-old Elsa, princess of Arendelle, possesses the ability to create ice and snow. One night while playing, she accidentally injures her five-year-old sister Anna with her powers causing a small part of her hair to turn white. The king and queen seek help from trolls, who heal Anna and remove her memories of her sister's magic. Their leader, Grand Pabbie, informs them that if Elsa had struck Anna's heart it would have been fatal. In order to protect Elsa and avoid her powers from endangering anyone, the royal family locks themselves away in their castle. Elsa, afraid of hurting Anna again, spends most of her time hiding in her room, creating a rift between the two sisters as they grow up. 10 years later, their parents are killed at sea during a storm.
Three years after the death of their parents, the people of Arendelle prepare for Elsa's coronation ceremony. Among the dignitaries is the Duke of Weselton, a tradesman who wants to exploit Arendelle for profit. Excited that the castle's gates will finally open to the public, Anna expresses her hopes of falling in love with someone. Strolling through the streets, she meets Prince Hans of the Southern Isles, and the pair quickly develops a mutual attraction for each other. Despite Elsa's concerns, the coronation goes off without incident, and the two sisters begin to bond at the reception party. During the reception, Hans proposes to Anna, who immediately accepts. Elsa, however, refuses to grant her blessing on the marriage, setting off an argument between the sisters, which culminates in Elsa's cryokinetic abilities being exposed to the guests.
Panicking, Elsa flees, inadvertently unleashing an eternal winter on the kingdom in the process. After coming to terms with her secret being out, she rejoices that she no longer has to be afraid of her abilities, building herself an ice palace and unknowingly bringing to life hers and Anna's childhood snowman, Olaf Anna sets out in search of Elsa, determined to return her to Arendelle, end the winter, and mend their relationship. While getting supplies at Oaken's Trading Post, she meets mountain man Kristoff and his reindeer Sven. Anna convinces Kristoff to guide her to the North Mountain. After an attack from wolves, the pair encounter Olaf who leads the group to Elsa's hideaway.
Arriving at her palace, Anna and Elsa are reunited, and the former attempts to persuade her elder sister to return. Elsa rebukes her, however, still harboring fears of hurting Anna. As Anna persists, Elsa becomes agitated and accidentally strikes Anna in the heart with her powers. Elsa, in desperation to get her little sister to leave, creates a giant snow creature that throws them out. After they outrun the snow monster, Kristoff notices that Anna's hair is turning white, and decides to take her back to his adoptive family of trolls. After a mix-up, she collapses and Grand Pabbie reveals that only an act of true love can thaw a frozen heart, or else Anna will die. Kristoff races back to Arendelle to get Anna to Hans, believing true love's kiss will save her.
Meanwhile, Hans, who had gone on a search for Anna, brings Elsa back to the castle, after she was knocked unconscious from an earlier skirmish with Weselton's men. Shackled in a cell for her protection, Hans pleads with her to undo the winter to no avail, as Elsa admits that she is unable to control her powers. Anna is returned to Hans, as she continues to freeze. She begs him to kiss her to break the curse, but Hans refuses, revealing his true plan of marrying Anna to seize control of Arendelle's throne. Leaving Anna to die, Hans goes after Elsa, framing her for her sister's apparent death.
Elsa escapes from prison and heads out into the blizzard on the fjord. Olaf comes to Anna's rescue and reveals Kristoff is in love with her. The two then travel out together onto the fjord to find him. Hans cuts Elsa off and tells her Anna is dead because of her actions. In Elsa's despair, the storm immediately stops, giving Kristoff and Anna the chance to reach each other. However Anna, seeing that Hans is about to assassinate Elsa, throws herself between the two and subsequently freezes solid, blocking the blow. After a few moments of grief from Elsa, Anna begins to thaw, as a result of her choice to sacrifice herself to save her sister, constituting an "act of true love".
Elsa realizes love is the key to controlling her powers and thaws the kingdom, while maintaining a personal microclimate for Olaf to prevent him from melting. Hans is sent back to the Southern Isles to face punishment, and Elsa cuts off trade with Weselton. Anna and Kristoff share a kiss. Elsa promises to never close the castle gates again and reconciles with Anna, and everyone celebrates.
Elsa was originally going to be what one would expect from a Disney villain but after the song 'Let It Go' was written in such a way as to create sympathy for her character (who wasn't in that draft Anna's sister) the creative direction shifted.