Lusa, a black bear cub in the zoo, promises Oka, a grizzly bear in the same zoo, to find her cub Toklo and protect him. After Oka dies, Lusa pretends to be sick so that her zookeepers will remove her from her enclosure for medical treatment. When the zookeepers take Lusa from her enclosure, Lusa runs away and climbs over a chain-link fence, leaving her zoo home to search for Toklo. When Lusa meets Toklo in the woods, he is accompanied by a shapeshifter named Ujurak. Ujurak spends most of his time as a young grizzly bear but occasionally shapeshifts into the form of another forest animal or a human. Toklo is not happy to see Lusa or to hear any news of his deceased mother, who abandoned him.
Ujurak says that he and Toklo are on a journey to a place where the bear spirits dance in the sky. Ujurak invites Lusa to join them on their journey. Lusa accepts, but Toklo is not pleased with her presence. After several days of traveling over the mountainous terrain, which Ujurak calls Sky Ridge, the three cubs are followed by a pack of wolves. Ujurak transforms himself into a deer in order to distract the wolves from following Toklo and Lusa. Ujurak soon rejoins his friends, and they proceed on their journey.
A second storyline is introduced: A polar bear cub named Kallik survives the wreckage of a helicopter that was returning her to her habitat in the wild. She collapses from the shock of the crash, and she hears the voice of her deceased mother telling her that she is a strong cub. Kallik listens to the encouragement of her mother's spirit and continues her journey to find her brother, Taqqiq. Kallik believes that the spirits of other dead bears are leading her to travel inland, away from the sea, to find her brother.
Toklo and Lusa travel onward, led by Ujurak's intuition. When at a crossroads, Lusa asks to pick their next direction. When she does, Ujurak confirms that her decision to cross a river is a wise one. As they cross, Toklo nearly drowns because he is not used to deep water. Lusa teaches Toklo how to swim. The three cubs discover a human settlement and look for food. Toklo kills a squirrel for himself and kills a grouse for Lusa and Ujurak to eat.
Kallik sees a helicopter landing, and she watches the release of another set of polar bears — a mother and her two cubs — into the wild. Kallik's loneliness leads her to follow the scent of the other bears.
Lusa looks through garbage cans for food but is scared away by a human before she can steal anything more than a chicken carcass. Toklo tells Lusa that she is not a proper bear because she steals instead of hunts. Lusa feels depressed and wonders if Toklo and Ujurak would have an easier trip if she were no longer with them. Toklo kills a sheep for the three cubs to share.
The next day, Lusa asks Toklo why he hates to swim. Toklo admits that he is afraid of the water and thinks the deceased spirits of his mother and his brother, Tobi, are trying to drag him under to join them. Lusa tells Toklo the story of how Oka was brought to the zoo where Lusa lived. Oka was sorry for abandoning him. Toklo is still angry and not ready to forgive his mother.
Toklo, Lusa and Ujurak cross over a manmade stone bridge, since Toklo does not want to swim across the river. Lusa is unfamiliar with the vegetation of the forest and nearly eats poisonous berries before Toklo stops her. The cubs venture on a highway, and a car injures Ujurak. Toklo feels guilty for allowing him to be hurt.
As Kallik is walking, she has a vision of her deceased mother and her missing brother, Taqqiq. She comes back to her senses as she runs into an adult polar bear that she doesn't know. The bear is following the Claw Path to find a special lake, which is a meeting place for bears. Kallik asks to travel with the older bear, but the bear refuses her company, saying that polar bears were meant to live alone and travel alone. Kallik discovers that an Arctic fox has been following her, and she allows the creature to curl up next to her for warmth at night.
Lusa finds the herbs Ujurak needs to help heal his injury. Toklo kills a rabbit and tells Lusa that she has earned her share of the meat because she helped Ujurak. The cubs arrive at a lake surrounded by grizzly bears. Ujurak says that this is the place the bear spirits were leading them all along. The grizzly bears are unfriendly toward Lusa, who is considered an outsider because she is a black bear. Lusa leaves Ujurak and Toklo to join a nearby group of black bears. Lusa and the two grizzly cubs promise to meet again sometime.
When Kallik wakes up, she finds that her Arctic fox friend has killed a rabbit for her. In return, Kallik kills a snow goose for the fox. Kallik and the fox have to part ways, but she is glad that they were able to be friends for a short while. Kallik reaches her destination, Great Bear Lake.
Toklo speaks to an old bear at the lake, who tells him that there are few fish in the lake; so all the bears are hungry. The oldest bear present, Oogrook, believes that one grizzly bear must swim unaccompanied to a dangerous place called Pawprint Island. This symbolic trip will make the bear spirits bring the fish back. A mean bear named Shoteka says that Toklo should be the bear to make the difficult trip, and the other bears agree that Toklo is the right bear for the task. Toklo begins the swim to the island.
Kallik overhears other polar bears talking about how the ice melts sooner every year. The bears have gathered at the lake on the longest day of the year to ask the ice spirits to bring the ice back. Some of the older polar bears are kind to Kallik and share their stories with her. Kallik is surprised to hear that brown and black bears are gathering on the other side of the lake. She thinks that polar bears are the only kind of bears in existence. Kallik catches a fish from the lake, but another cub steals it from her. The thieving cub turns out to be Taqqiq, Kallik's long-lost brother. Taqqiq does not recognize Kallik at first, but when he understands that she is his sister, he walks away from her. Taqqiq prefers the company of his new friends and does not want to be with his sister.
Lusa meets another black bear cub named Miki and becomes friends with him. Miki is surprised that Lusa arrived at the lake in the company of grizzlies, because he believes that grizzlies eat black bears.
Toklo has trouble swimming to Pawprint Island, but the spirits of his mother and brother encourage him and push him to the surface of the water when he begins to sink. Toklo explores Pawprint Island and discovers that the mean bear Shoteka has followed him there. Shoteka and Toklo fight and inflict serious wounds on each other. When Toklo wins, Shoteka swims away, promising to get his revenge on Toklo someday. Toklo is proud of himself for defending his territory.
Kallik overhears Taqqiq and his friends planning to raid the black bears' territory on the other side of the lake. They believe the black bears have more to eat, and a show of force will intimidate them. Taqqiq and his friends attack Miki and drag him back to the polar bears' territory. Lusa has trouble convincing any of the older black bears to help rescue Miki because they are afraid of fighting the larger, stronger polar bears. The young polar bears chase Lusa, and without knowing it, she swims to Pawprint Island. She meets Toklo there, but Toklo does not want to leave his island to help Lusa rescue her friend.
Kallik is horrified when Taqqiq returns from his raid carrying a black bear cub. Kallik and Taqqiq have a fierce argument about whether the polar bears should invade the black bears' territory and live off the food in the forest instead of going back to the ice. Taqqiq tells Kallik to leave and never come back; he chases her into the lake. Kallik nearly drowns, but Lusa saves her. Lusa has come to rescue Miki, and Kallik agrees to help her.
On Pawprint Island, Toklo remembers how brave Lusa was to leave her home in the zoo and come to deliver his mother's last message to him. He decides he owes Lusa a favor. Ujurak joins Toklo, and the two of them go to Lusa's and Kallik's aid.
Kallik tries reasoning with Taqqiq, and she reminds him of how it feels to be a small, frightened cub. While the polar bear guards are distracted, Miki runs away. One mean polar bear named Salik begins to chase Lusa, but Ujurak and Toklo come to Lusa's rescue. Ujurak and Toklo are losing their fight because the young polar bears are helping Salik, until Taqqiq defends the young grizzlies against his fellow polar bears. Taqqiq says that it is not right for polar bears to bully black bears. The polar bears do not have to steal food because, when the ice returns, they will have plenty of fish to eat.
Lusa oversees Miki's safe return to the black bears, but she tells him that she is not returning. Lusa's destiny is to keep travelling with Ujurak and Toklo to see the bear spirits dancing over the Endless Ice.
Toklo returns to the grizzlies' territory to tell them about his experiences on Pawprint Island. On his way, Toklo kills a salmon, and the other bears see this as a sign that his symbolic journey was successful and that the bear spirits are blessing them with food again. Toklo rejoins Lusa and Ujurak. Kallik and Taqqiq, who have decided to go on the journey to the Endless Ice, join, too. The five bears start their journey.