Princess Elsa of Arendelle possesses magical powers that allow her to control and create ice and snow, often using them to play with her younger sister, Anna. After Elsa accidentally injures Anna with her magic, their parents, the King and Queen, take both siblings to a colony of trolls led by Grand Pabbie. He heals Anna, but alters her memories so that she forgets about Elsa's magic. Grand Pabbie warns Elsa that she must learn to control her powers, and that fear will be her greatest enemy. The King and Queen isolate both sisters within the castle, closing the castle gates to their subjects. In an effort to protect her sister from her increasingly unpredictable powers, Elsa ceases all contact with Anna, creating a rift between them. When the sisters are teenagers, their parents are lost at sea during a storm.
Following her 21st birthday, Elsa is to be crowned queen of Arendelle. She is afraid that the kingdom's citizens might find out about her powers and fear her. The castle gates open to the public and visiting dignitaries for the first time in years. Among them are the scheming Duke of Weselton and the dashing Prince Hans of the Southern Isles, with whom Anna falls in love at first sight. Elsa's coronation takes place without incident, but she still remains distant from Anna. Anna and Hans develop a romantic connection during the coronation festivities, and he impulsively proposes to her, but Elsa objects when they seek her blessing. Hurt and confused, Anna protests, begging Elsa to explain her fear and isolation. The emotional strain causes Elsa to accidentally unleash her powers before the court. Branded a monster by the Duke, Elsa flees to the North Mountain, where she finally acknowledges her powers, building an ice palace in which to live a hermit life. In the process, her magic unintentionally engulfs Arendelle in an eternal winter
Anna ventures out to find Elsa and end the winter, leaving Hans in command. She gets lost, collecting supplies at Wandering Oaken's shop. She meets an iceman named Kristoff and his reindeer, Sven, convincing them to take her to the mountains. An attack by wolves leads to Kristoff's sleigh being destroyed. On foot, they meet Olaf, a cheerful snowman brought to life unknowingly by Elsa, who offers to lead them to her. When Anna's horse returns to Arendelle without her, Hans sets out to find Anna and Elsa, accompanied by the Duke's minions, who have secret orders to kill Elsa.
Reaching the ice palace, Anna meets Elsa. When Anna reveals what has become of Arendelle, a horrified Elsa confesses she does not know how to undo her magic. Her fear causes her powers to manifest themselves once more and she accidentally freezes Anna's heart, seriously injuring her. Elsa then creates a giant snow monster named Marshmallow, who chases Anna, Kristoff, and Olaf away. Realizing the effects of Elsa's spell on Anna, Kristoff takes her to the trolls, his adoptive family. Grand Pabbie reveals that Anna will freeze solid unless "an act of true love" reverses the spell.
Kristoff races Anna back home so Hans can give her true love's kiss. Hans and his men reach Elsa's palace, defeating Marshmallow and capturing Elsa. Anna is delivered to Hans, but rather than kissing her, Hans instead reveals that he has actually been plotting to seize the throne of Arendelle by eliminating both sisters. Hans locks Anna in a room to die, and then manipulates the dignitaries into believing that Elsa killed her, but not before they were married. He orders the queen's execution, only to discover she has escaped her detention cell.
Olaf frees Anna, and they venture into the blizzard outside to meet Kristoff, who Olaf has revealed is in love with her. Hans confronts Elsa outside, claiming that she killed Anna, causing Elsa to break down and abruptly stop the storm. Anna spots Hans about to kill Elsa; she leaps in the way and freezes solid, stopping Hans. Devastated, Elsa hugs and mourns over her sister, who thaws out, her heroism constituting "an act of true love".
Realizing that love is the key to controlling her magic, Elsa ends the winter and gives Olaf his own snow flurry to survive the warmer climate. Hans is arrested and exiled from the kingdom for his attempted assassination, while the Duke's trade links with Arendelle are cut off. Anna gives Kristoff a new sleigh and the two kiss. The sisters are reunited and Elsa promises never to lock the castle gates again.
In a post credits scene, Marshmallow finds Elsa's discarded crown. Marshmallow places the crown on top of its head and smiles.
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