Elsa and Anna are two sister princesses. Elsa was born with magical powers that allow her to create snow in thin air. One day, while the two are playing, Elsa accidentally hits Anna's head with her powers. Anna is taken to trolls, who heal Anna but erase the memory of Elsa's powers. Elsa is told to hide her powers from the world. Years later, their parents die when their ship sinks.
Three years afterwards, Elsa is to be coronated as queen. Anna, who still has memories of having fun with her sister, is excited to finally experience fun again, after years of isolation. As the front gates are thrown open, Anna explores her kingdom and happens upon a young prince named Hans. The two fall in love, and Hans asks Anna to marry him.
Elsa is coronated as queen, but becomes upset when Anna asks for her blessing in her marriage. Elsa refuses, on the basis that true love cannot exist after mere hours of knowing someone. Anna, tired of being rejected by her sister, confronts her, and Elsa unwittingly reveals her snow powers. Instantly labeled a "monster" by bystanders, Elsa flees the kingdom. As she leaves, she unknowingly leaves it in heavy snow.
Anna blames herself for Elsa's problems and opts to follow her. She leaves the kingdom with Hans in charge before persuing her sister. Along the way, she happens upon another young man, Kristoff, whose best friend is a reindeer named Sven. She recruits Kristoff to help her find Elsa.
Elsa, meanwhile, has let go of the restraints others have put on her, and has unleased her snow powers to their fullest extent. She creates an ice castle at the top of a mountain and, accidentally, a living snowman named Olaf. Anna and Kristoff find Olaf, who dreams of knowing what "warmth" is--and apparently doens't know that it would mean him melting if he found out.
The group finds Elsa's castle, and Anna decides it would be better to talk with her alone. During their conversation, Elsa reveals she never intends to return to the kingdom, since she can be herself in isolation, and thus be happy. Anna, however, wishes for Elsa to return, because she doesn't care what others say about her. Besides that, the kingdom is in deep, deep, deep, deep snow, and Elsa needs to thaw it.
Elsa, however, doesn't know how, and in her frustration, accidentally unleases more snow powers. This time, it hits Anna's heart. In a final effort to disway Anna and her friends, Elsa creates a snow monster, who scares them away. After they escape, Kristoff notices Anna's hair is turning white, and realizes that Elsa has frozen her heart. Knowing the only way to save Anna is to take her to the trolls, Kristoff sets course in that direction.
Meanwhile, Anna's horse has returned to the kingdom, and Hans has taken this as a sign that she is in trouble. He organizes a search party to find Anna and Elsa, and they rush to the ice castle. When they get there, they defeat the snow monster and capture Elsa, whose powers continue to grow out of control.
Anna is taken to the trolls, who inform Kristoff that she cannot be saved with their powers. Only an act of true love can save her. Kristoff and Anna hurriedly rush back to the kingdom, so that she can kiss Hans, her true love, and be saved.
Once they arrive, Kristoff ensures that Anna is taken to Hans, before he takes Sven and begins to leave. Anna is left alone with Hans, but instead of kissing her, Hans reveals that he plans to overtake the kingdom. He leaves Anna to freeze to death, and intends to blame it on Elsa, so he has a reason to execute her and take over the kingdom.
However, Olaf finds Anna and builds a fire to keep her warm. He discovers what warmth is, and equates it to love--like Kristoff's love for Anna. Just as he's speaking, he looks out the window and spots Kristoff and Sven running back to the kingdom. Olaf helps Anna escape the palace so she can kiss Kristoff--who they both know is her real true love.
Meanwhile, Elsa has escaped prison and is trying to make her way back to her ice castle. Her powers are now out of control, and a blizzard has enveloped the kingdom. Hans, however, finds her and reveals to her that she froze Anna's heart. Shocked by this revelation, Elsa freezes the snow, which causes a break in the blizzard.
Kristoff spots Anna and runs to her, but at the same time, Anna sees Hans about to kill Elsa. With a last-second decision, she throws herself between Hans and Elsa just as her heart freezes solid, which turns the rest of her body into ice. Hans is thrown back as everyone else begins to weep for Anna.
However, Anna's act of true love--the love she has for her sister--is enough to warm up her heart. Her body unfreezes and her hair turns back to its original color. Elsa realizes that there is a way to control her powers: the power of love. She unfreezes the kingdom, while at the same time giving Olaf his own personal bubble of cold, so he can continue to live.
In the end, Elsa rules over a happy Arendelle, and Kristoff and Anna are deep in love. Hans is sent back to his own kingdom, and Olaf is happily enjoying Spring.