Sitka loves his younger brothers and would do anything for them, the movie begins with Kenai's coming of age ceremony where each tribe member receives an animal totem which represents their personalities and he happens to be a bear, which he hates because they steal food from them and Denahi teases him for it, angering him. Sitka then tells Kenai of his own ceremony and how his totem is the eagle, which represents leadership and guidance. Moments later, they find that a bear has stolen some fish from them because Kenai didn't secure the ropes on the basket and he recklessly goes after the bear. Sitka and Denahi follow him to make sure he doesn't end up getting killed.
Kenai is knocked to the ground and Denahi defends him, but the bear knocks the spear out of his hands and it looks like it's over for them when Sitka, deciding there is no other option, stabs the ice around the bear with his spear, causing the ice to crack and break off, causing the bear to fall with him. Kenai and Denahi search for him, but only find his coat and his spirit assumes the form of a bald eagle. Later that night, a funeral is held for Sitka and Kenai is enraged that the bear lived while Sitka died and feels the bear responsible for what happened, so he goes out to kill it to avenge Sitka. After a battle which he succeeds, a bald eagle appears and morphs into Sitka in an ethereal form. He then approaches Kenai and is very disappointed in his brother's actions. Sitka felt that it wasn't necessary for Kenai to kill the bear because he willingly gave his life to protect them, so he punished Kenai by turning him into a bear.
Thinking the bear killed Kenai, Denahi decides to avenge his death and tracks him down and Kenai escapes and has to look after the bear's cub, Koda, whom he finds annoying at first, but eventually grows to love him and feels guilt after learning that he is the son of the bear he killed. At the end of the movie, upon seeing Kenai nearly give his life for Koda, Sitka decides to change him back into a human and visits him and Denahi in his human form before departing to the spirit world but, before he leaves, Kenai wishes to be a bear again so that he can look after Koda because he can't talk to him in human form. And with that, Sitka and Koda's mother returns to the afterlife.
He does make a cameo in his eagle form near the end of Brother Bear 2 when the Great Spirits transform Nita into a bear so she can be with Kenai and Koda (mainly Kenai).