The story opens with the townspeople of Arendelle celebrating the royal family. Princess Elsa is born with magical power to create ice and snow. While playing, she accidentally harms her younger sister, Anna. To save Anna, the King and Queen seek the help of the mystical Hidden Folk, who heal Anna but erase her memories of Elsa's magic. To protect both girls, the King and Queen close the castle gates and isolate Elsa until she can control her powers. Confused by the sudden distance, Anna spends years trying to reconnect with her sister ("Do You Want to Build a Snowman?"). 

Years later, after their parents are tragically lost at sea, Elsa comes of age to be crowned Queen. The gates are opened for coronation day, bringing visiting royalty like the plotting Prince Hans of the Southern Isles and the Duke of Weselton. Anna meets Hans and instantly falls in love, asking Elsa for her blessing to marry. Overwhelmed by the pressure and Anna's sudden demands, Elsa loses control of her emotions—and her powers—accidentally exposing her magic to the horrified kingdom and setting off an eternal winter. Terrified, Elsa flees into the mountains ("Let It Go").

Anna bravely sets off into the freezing wilderness to find Elsa and bring back summer. Along the way, she teams up with a rugged mountain man named Kristoff, his loyal reindeer Sven, and a whimsical, living snowman named Olaf. When they find Elsa's ice palace, Anna tries to convince her sister to return. Defensive and panicked, Elsa accidentally strikes Anna in the heart with a blast of ice magic. 

Kristoff rushes Anna to the Hidden Folk, who reveal that only "an act of true love" can thaw a frozen heart. Believing a kiss from her fiancé Hans is the cure, they race back to Arendelle. However, Hans betrays Anna, revealing he only wooed her to seize the throne, and locks her away to die.

Elsa is captured and brought back to the castle but manages to break free into the blizzard. Just as Hans raises his sword to strike Elsa down, a dying Anna uses her remaining strength to leap between them, freezing solid into solid ice just in time to block the blow. Heartbroken, Elsa weeps and embraces her sister's frozen form. Elsa's grief and pure love for Anna trigger the magic required to thaw her heart, proving that the "act of true love" was sisterly devotion. Summer is restored to Arendelle, the gates are opened permanently, and the sisters vow to fill the kingdom with light and love.