Leo, Masane Tsukayama is an adult who learns that his mate Lyre, Chieko Baisho, has just given birth to twin cubs named Lune and Lukio. After a grand celebration, the film shifts to a bustling city where a man named Ham Egg travels from jeweler to jeweler trying to hawk a mysterious stone he found in the Bajalu Jungle. When his pitches are rejected, the jewelers let someone know where he’s heading, and soon Ham Egg is hauled away in a black car by intimidating men in dark suits. The stone he carries isn’t just a curiosity—it’s the legendary Moonlight Stone, a potential power source that could stave off an looming energy crisis threatening the world.
A scientific outfit led by Dr. Plus and Dr. Minus recruits Ham Egg to lead them to the Moonlight Stone’s source so it can be salvaged. Ham Egg, driven by greed, wants to control the discovery and insists on leading the expedition, while Dr. Plus knows Ham Egg’s illegal poaching history and tries to steer the plan toward safety and accountability. Accompanying Ham Egg are Mr. Lemonade, a steady recorder of events for the organization, and Dr. Moustache, who is already stationed in the jungle and unafraid to challenge the humans’ intrusive approach to nature.
Back in the jungle, the humans’ presence grates on the wildlife. Dr. Moustache continually clashes with Ham Egg and his crew over basic respect for the forest and its inhabitants, while Mr. Lemonade keeps a meticulous journal of every action. Leo’s son Lune grows increasingly curious about the human world, and Leo makes it his mission to shield the jungle’s animals from poachers, disease, and the growing unrest among the creatures—led by Pagoola, the African elephant who serves as a fierce but wary leader. Earth Mother, a benevolent mammoth elder from Mount Moon, inspires Leo to persevere in the face of mounting danger and ecological distress.
Lune eventually travels away from his father’s protection and slips into the wider world by river, ending up in a nearby city where he is discovered by fishers and sold to a circus. There, an earnest girl named Mary takes him under her wing and befriends a clever rat named Jack. A jagged thread of hope appears when a jungle-stork arrives and warns of a spreading disease that is killing many animals. The help Mary offers culminates in a crucial moment when a devastating fire breaks out at the circus, prompting Lune to rally the other animals to help extinguish the flames. After a heartfelt farewell, Lune sneaks onto a boat bound for Africa, hoping to reunite with his family.
Meanwhile, Lyre is stricken by the same deadly illness afflicting the jungle, and Lukio falls ill as well. Dr. Moustache and Dr. Lemonade work to find a cure, and their efforts do bear fruit, demonstrating that humans can indeed be friends to animals when motivated by genuine care and responsibility. In the jungle, Leo makes the choice to lead Dr. Moustache and Mr. Lemonade toward the mountain’s center, where a cluster of Moonlight Stones pulses with otherworldly energy. Earth Mother guides them and helps fend off a pack of dire wolves along the treacherous ascent. The ascent is perilous, and many hunters lose their way as the path narrows toward the luminous stones.
As the trio reaches the stones, Ham Egg bursts onto the scene and fires at them. In a final act of sacrifice, Mr. Lemonade jumps in front of the shot and takes the bullet for Leo and the others. They uncover the Moonlight Stones, but Ham Egg—blinded by greed—seizes one and swallows it to keep it from others, only to die soon after when the stone poisons him. Dr. Moustache realizes the burden the Moonlight Stone has placed on the world and, with Leo by his side, they retreat down the mountain. A bleak twist of fate reveals that Leo has gone blind, a consequence of the stone’s influence, and Moustache discards the stone, believing it has already caused enough harm.
The story circles back to Lune’s return from the human world. Dr. Moustache takes the cub to see Leo’s pelt, and Leo dies along the journey, stabbing himself with Dr. Moustache’s knife so that Moustache can use Leo’s fur to tell humans what they need to hear. In a poignant moment, Moustache assures Lune: “Your father was as wise as he was brave, and you should know what he did to save the world.” Then the two head into a meadow, where they gaze at the clouds forming the shape of Leo, a final image of a king who gave his life to safeguard the land.
