When Anna was a young girl, she and Elsa would often play with each other, mainly using Elsa's powers over ice and snow. However, when Anna was five, Elsa struck her in the head unintentionally. However, she recovered with the help of a clan of trolls. However, the trolls readjusted her mind so she had no memories of Elsa's powers. After the incident, their parents kept the two girls isolated. However, Anna repeatedly attempted to reach out to Elsa, but was shut out every time. Several years afterwards, their parents passed away in a storm.
Three years after their death, Elsa was to be crowned queen of Arendelle. Anna was ecstatic at the gates being opened so she could socialize with the townspeople, but Elsa was secretly frightened at her powers being revealed to the public. Shortly after the gates were opened, Anna met Prince Hans and became infatuated with him. Anna later joined Elsa at her coronation where she was crowned queen.
At the celebration party, Anna and Elsa met The Duke of Weselton, who tried to dance with Elsa. However, Elsa convinced him to dance with a reluctant Anna instead. Afterwards, Anna mentioned bonding with Elsa, but the queen blew off the idea, leaving a despondent Anna to have another encounter with Hans, and the two had a duet which ended with Hans proposing to Anna, who readily accepted. However, when the two asked Elsa for her blessing, she refused and informed Anna that she could not marry Hans. Upset, Anna instigated an argument between the two that resulted in Elsa revealing her powers to those present in the ballroom.
Startled, Elsa fled out of the kingdom and her powers were further revealed to the townspeople, and the Duke immediately labels Elsa a monster. Elsa then flees into the mountains, and unintentionally curses Arendelle, plunging it into an eternal winter. Anna, feeling responsible, then sets off to return Elsa to Arendelle so she can bring back summer and puts Hans in charge in her absence.
Anna later runs into Oaken's Trading Post, and meets an ice harvester named Kristoff and his reindeer Sven. The two accompany Anna on her journey, but a wolf attack results in their sled getting destroyed, to which Anna promises to get them a new one if they help guide her to the mountains. As they near the mountains, Olaf, a snowman who had been given life by Elsa, runs into the three and guides them into Elsa's castle.
Upon reaching Elsa's castle, Anna enters the palace and reunites with Elsa. However, Elsa refuses to return to Anna, and upon being informed of the curse she had cast on Arendelle, she loses control of her powers and strikes Anna in the heart. Elsa then creates a giant snow golem named Marshmallow, who throws the group out of the castle. However, an enraged Anna hits Marshmallow with a snowball, to which he attacks the group and results in them falling off a mountain.
Kristoff then notices Anna hair changing white, which is a result of Elsa striking her in the heart. Kristoff brings Anna to the troll clan, who inform Anna that she will eventually die, but an act of true love can thaw her frozen heart. Believing Hans can save her, Kristoff returns a weakened Anna to Arendelle, where she imparts Hans on the situation and asks him to kiss her. However, Hans rejects her, revealing his true intentions to marry into the throne and murder Elsa, and after locking Anna in the room, he sets off to execute Elsa, who had previously been captured.
Olaf enters the room and starts a fire, slowing down Anna's freezing process. He also tells her that Kristoff loves her, giving her to idea that kissing him will save her. However, Elsa escapes captivity and causes a blizzard. Olaf frees Anna and the two escape the castle and try to find Kristoff on the fjord. However, Olaf is blown away by the blizzard and Anna's freezing process is sped up by the blizzard. However, Elsa is informed by Hans that Anna is dead, causing her to break down and the blizzard to subside, giving Anna and Kristoff the chance to reach each other. However, Anna notices Hans about to stab Elsa and chooses to save Elsa by deflecting the blow just as she freezes solid.
Elsa grieves Anna's supposed death, but her sacrifice for her sister constitutes as an act of true love, and she returns to her normal state. Elsa then learns to control her powers and thaws the eternal winter, restoring summer. Anna also gets revenge on Hans by punching him off the ship their standing on. Hans is later shipped back to the Southern Isles to face punishment for his actions, while Anna and Elsa reconcile and Anna starts a relationship with Kristoff.