The people of Arendelle long lived in harmony with the Northuldrans. It is said, however, the Northuldra grew too powerful and caused an ambush during a peaceful celebration. The people of Arendelle believe that in this battle Chief Bierrán killed the merciful and loved King Runeard, and a great fog descended over the Enchanted Forest, allowing no one in or out. Prince Agnarr, only a child, returns to the Kingdom to rule. Meanwhile the Chief's granddaughter, Iduna, finds herself on the other side of the fog. This is a story of their journey, forever changed by the actions of their ancestors.