After the citizens of Arendelle are forced to evacuate their kingdom following a mysterious supernatural attack, Queen Elsa, Princess Anna, her boyfriend, Kristoff, and their sidekicks head into an enchanted forest, where Anna and Elsa’s late father encountered the Northuldra tribe as a young boy. As he explains to his daughters in a flashback, he had traveled to the woods with his father—Elsa and Anna’s grandfather—to present the Northuldra a dam, apparently a gift of peace.

It turns out that the generosity of the sisters’ grandfather was actually an act of sabotage meant to harm the tribe’s lands. The grandfather then attacked a Northuldra leader in cold blood when his real plan was found out, and in response to the resulting battle, the Enchanted Forest where the Northuldra live covered itself in an impenetrable mist. Decades later, Elsa, Anna, and company are allowed to enter so that the sisters can find out the truth about their ancestor’s misdeeds.

While they’re in there, Anna and Elsa also discover another twist: They’re half-Northuldran. Their mother, it turns out, was a tribe member who saved their father during the battle, escaped with him out of the forest, and later married him.

When Anna discovers that her royal ancestors committed brutalities against their neighbors, the princess decides to destroy the dam to make things right with the tribe—even though it means flooding Arendelle.

When the dam is destroyed and the wave is about to hit Arendelle, Elsa freezes the surge so that the kingdom is safe.