In a winter landscape, we see ice harvesters using saws and hooks to cut blocks of ice from a lake, chanting as they work ("Frozen Heart"). They load the ice blocks onto their wagon and then head into town. A young boy named Kristoff (Jonathan Groff) is seen working alongside them (not very expertly) accompanied by his reindeer Sven (sounds supplied by Jonathan Groff). They imitate the ice harvesters with a single cubic block of ice and a small child's sled.
That night, the action shifts to the Kingdom of Arendelle, where the king and queen have two daughters: the elder, named Elsa (Eva Bella), and the younger, named Anna (Livvy Stubenrauch). Elsa was born with a power to create and manipulate snow and ice, but Arendelle is not a place where magic is valued.
That night, Anna wakes Elsa, wanting to play. Elsa allows herself to be persuaded when Anna suggests they build a snowman. Elsa turns the ballroom into a winter playground, complete with a snowman she calls Olaf, and claiming he likes warm hugs. The girls play gleefully until Anna makes a leap Elsa wasn't prepared for and the blast of power meant to create a pile of snow hits Anna in the head, knocking her out and turning several strands of her hair white. The king and queen rush both girls to a group of trolls in the kingdom's mountains.
Kristoff happens to be in the woods with Sven still pulling the block of ice when Anna and Elsa's parents ride by at speed, Elsa's powers leaving a trail of ice behind the horses. He and Sven watch from afar and hide behind some boulders as the king and queen enter a clearing full of rocks. The king asks for help, and seconds later, all of the boulders start rolling into a circle around the king, queen, Elsa and the unconscious Anna. They then uncurl themselves, revealing them to be trolls. In fact, the "boulder" Kristoff and Sven are watching from behind is a troll named Bulda, who immediately decides to adopt the visitors after Sven licks her.
As Kristoff and Sven watch, Pabbie (Ciarán Hinds), the leader of the trolls, shows up and asks the king if Elsa was born or cursed with her abilities. He observes that Anna is lucky she was hit in the head, as a hit to the heart would have been fatal. He advises the family that it might be best to not have Elsa use her powers around Anna, and alters Anna's memories so she has no knowledge of her sister's powers, remembering only the fun they've had. Pabbie warns Elsa that her powers will grow, and they'll be a risk to her if she cannot learn to control them, as fear will be her greatest enemy.
Subsequently, the palace is closed off to visitors. Staffing is reduced to a minimum. The girls' parents separate the two, and having no memory of what has occurred, Anna is unable to comprehend why Elsa is not allowed to play with her. She often comes to Elsa's closed door and tries to coax her out by asking her if she wants to build a snowman ("Do You Want to Build a Snowman?"). As a further precaution, the sisters are also kept from leaving the castle.
While Anna's life is dull but normal, Elsa's powers grow stronger as she matures. Her father cautions her to wear gloves to keep her icy magic in check, and to conceal her feelings, because strong emotions seem to cause her powers to manifest in unexpected ways.
A few years later, the now teenaged princesses lose their parents to a storm at sea. Anna goes again to Elsa's door, pleading for consolation from her only remaining family member. But Elsa, though she sits sadly on the the other side of the door, refuses to communicate with Anna.