The Entire Frozen Timeline Explained

Elsa and Anna's childhood bond becomes complicated when a playtime goes wrong and Elsa accidentally hurts Anna with her ice powers. Agnarr and Iduna take the unconscious Anna to the Valley of the Living Rock, a land inhabited by a close-knit community of magical rock trolls. Pabbie, the elder troll leader, suggests that in order to prevent this from ever happening again, Anna's memories of Elsa's powers should be erased. He then heals Anna while doing so. To further ensure that neither Anna nor the villagers learn of Elsa's abilities, the castle's gates are locked and she is forced to confine herself to her room with the drapes drawn. When Elsa has trouble managing her powers, her father occasionally gives her a "conceal, don't feel" speech (which, as we're about to discover, isn't the best advice a parent could give their child).

Meanwhile, a young Kristoff earns a living picking ice (raising the question of how lenient Arendelle's child labor regulations are). He travels to the Valley of the Living Rock with his reindeer (and closest friend) Sven, where he's adopted as a part of the family and nurtured as their own. 

As they grow older, Elsa consistently rebuffs Anna's attempts to spend time with her while trying (and failing) to control her powers inside the room. She even distances herself from her parents, who have been her only point of contact, out of fear of accidentally hurting them. Agnarr and Iduna embark on a voyage through the Southern Sea to Ahtohallan, a mythical island they believe to be connected to Elsa's ice powers. However, the two perish in the voyage, leaving Elsa and Anna orphaned and more disconnected than ever.