The mystery unravels when Anna and Elsa find their parents' wrecked ship in the Dark Sea near the Enchanted Forest, learning Agnarr and Iduna died while trying to reach Ahtohallan to uncover the origin of Elsa's powers. Elsa crosses the Dark Sea by taming the Nokk and enters Ahtohallan, discovering that nature gifted her powers because their Northuldra mother saved their Arendellian father during battle. She also uncovers King Runeard's treachery: he built the dam to suppress the Northuldra and initiated the war by ambushing their leader. Elsa travels too deep into Ahtohallan to send this revelation to Anna, freezing solid in the process.
As Elsa freezes, Olaf dissolves in Anna's arms. Overcoming her despair, Anna lures the giant Earth Giants into hurling boulders at the dam, shattering it and releasing the floodwaters toward Arendelle. Breaking the dam dispels the forest mist and revives Elsa, who races on the Nokk to construct an ice barrier that shields Arendelle from destruction. Anna ascends as the Queen of Arendelle and accepts Kristoff's proposal, while Elsa embraces her role as the Fifth Spirit, remaining in the Enchanted Forest with the Northuldra while staying connected to her sister.