Elsa, princess of Arendelle and heiress to the throne, is born with the ability to create and control ice and snow. As a child, she uses her abilities to create a winter wonderland to play in with her younger sister and best friend, Princess Anna. One night, Elsa accidentally harms Anna with her powers. The king and queen of Arendelle hurriedly take Anna to a tribe of mountain trolls to be healed. While healing Anna, the trolls inform the royals present that Elsa's abilities will grow, becoming both beautiful and very dangerous so she must learn to control them. While the trolls erase Anna's memory of the incident and of her elder sister's powers in general, Elsa is traumatized by the event. The king and queen take steps to control and hide Elsa's ice powers: the castle gates are locked, Elsa is shut away in her bedroom for most of the time, she is given gloves to help suppress her powers and is told to hold in her emotions as well. Nonetheless her powers continue to grow even stronger and so she becomes fearful of harming those she cares about most. Meanwhile, her sister Anna is less happy and confused by the loss of contact with her elder sister and tries, without success, to coax her out of her room. When the sisters grow into teenagers, the ship in which the king and queen are sailing is capsized in a storm and they drown, leaving Anna and Elsa feeling even more lonely.
Three years pass, and Elsa, now of age, is set to formally succeed her father as the monarch of Arendelle. Though she is afraid of opening the castle to the large crowds, her coronation goes on relatively peacefully. However, at the reception party, Anna asks for Elsa's blessing to marry Prince Hans of the Southern Isles, whom Anna had met earlier that day. Elsa refuses to bless Hans's offer to marry Anna; he is someone she barely knows, prompting an argument between the two. Being so upset Elsa accidentally reveals her power. Upon the guests' and her subjects' horror and being accused of sorcery and called a monster, Elsa flees the castle and retreats into the icy mountains. In the process, her emotions unleash an "eternal" winter throughout Arendelle. While there, she decides to embrace, finally, all her powers and builds an enormous ice palace where she believes she can live freely without fear of hurting people. She also rebuilds her childhood snowman, Olaf, and unknowingly brings him to life.
Anna, determined to find Elsa and bring her back, travels through the mountains, encountering Olaf and a mountain man named Kristoff. They reach the ice palace, where Anna attempts to persuade Elsa to return home and mend their relationship. When Elsa ultimately resists (due to her memory of hurting Anna as a child with her powers resurfacing ) Anna tells her about the state that Arendelle and all its people was left in. Horrified, Elsa lashes out and accidentally freezes Anna's heart. Now even more horrified at the prospect of hurting her sister and people with her powers Elsa forces Anna, Kristoff and Olaf out by creating a gigantic snow creature (called Marshmallow by Olaf), that is a symbol of her desire to be alone so as to be able to use her powers without hurting anyone. After this, her ice castle evidently becomes darker and more grotesque, reflecting her torment and re-ignited fears. Meanwhile, Anna becomes weaker day by day and Kristoff takes her back to the trolls, who tell them that only an "act of true love" can save her life.
Hans and a group of soldiers attack the now ugly ice palace. Elsa seizes two soldiers who attempt to assassinate her. Hans convinces her to spare them to prove that she is not a monster. However, she is knocked unconscious and taken to her castle's dungeon. Hans visits her and urges her to end the winter but she admits that she has no idea know how to. After he leaves, she is able to break free from the chains by freezing them and escapes the cell, though her fears trigger a massive blizzard. Anna returns to the castle, believing that a romantic kiss from Hans will be the "act of true love" to save her. Instead, he informs her that his offer of marriage (engagement) had been the first step of a plot to get him the throne of Arendelle. Olaf tells Anna that Kristoff is in love with her and she believes that his kiss will cure her. They rush to find Kristoff. Hans confronts Elsa and tells her that she has killed Anna. Devastated, Elsa collapses and the blizzard stops suddenly. Hans approaches her and swings his sword to kill her, but Anna turns away from an approaching Kristoff with her last bit of strength and blocks Hans' attack as she freezes solid.
Moments later, Anna begins to thaw, as her choice to save her sister rather than herself constituted the necessary "act of true love". Elsa realizes that love is the key to controlling her powers and is able to end the kingdom's eternal winter. Summer returns to Arendelle, Elsa regains the throne and is able to use and safely control her powers, while the sisters' bond is restored. She exiles Hans back to the Southern Isles to face punishment from his family and cuts off trade with the Duke of Weselton's town for his earlier behavior towards her.
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