Elsa and Anna, two sisters, had a strong sisterly bond which got ruined because Elsa hit her head with ice blast.
Anna went to go find her sister Elsa who was on the North mountains so she can save Allende who was frozen. Elsa is the only one that could unfreeze her.
Elsa made Olaf as she was leaving Arendelle. Anna discovered him on her search for Elsa and he stuck with her and helped her for the entire journey.
Anna and Kristoff encounter a pack of wolves and they end up losing their sled since they had to jump a big cliff to escape the wolves. They also met Olaf (who Elsa created) who helped them along the journey. They reach Elsa, but Anna has a hard time to get to talk to her and in the end Elsa accidentally hits Anna in the heart with an ice blast and Anna gets a frozen heart.
Kristoff and Anna go to the Trolls to seek help for Anna since she has a frozen heart now and it can cause her to die. The Trolls tell her that the only way to get rid of the frozen heart is by an act of true love. So Anna thinks about Hans since they were engaged and she "knows" he loves her. During this Hans was searching for Elsa and Elsa almost dies due to the guards of the Duke of Weselton. Hans brings Elsa back to Arendelle so she could bring back summer, but he had more of a wicked plan.
Anna goes back to Arendelle with Kristoff. She arrives and meets Hans there. She tells him she needs an act of true love to live so they go in for a kiss, but Hans pulls away and tells Anna his scheme. Anna is locked in the room. Olaf finds her and she realizes Kristoff is the way so she goes out even in her condition. Kristoff goes back as well for Anna. Hans finds Elsa, tells her Anna died because of her. Once Elsa is weakened because of the thought of her sister being dead, Hans goes in for the kill since Elsa didn't see it coming. Anna sees this and instead of going to Kristoff she went to Elsa and stops the sword from hitting her since she turned into solid ice.
Elsa sees Anna frozen solid so she hugs her as she cries. Slowly Anna begins to unfreeze since the hug from Elsa was a form of true love. There and then Elsa and Anna realize that true love was the solution to everything.
Elsa returns to Arendelle. Elsa reunites with Anna.