A lion cub finds a music box in plane wreckage and becomes obsessed with the humans who made it, while his father Leo tries to protect a jungle that doesn't want protection from anyone. The movie moves at the pace of a fable, more interested in what curiosity costs than in spectacle. Tezuka builds something genuinely strange here: a story about inheritance and refusal, where the real conflict isn't between hero and villain but between a father trying to pass down a world and a son who wants to understand the creatures destroying it first.

Leo, king of the jungle, lives at the foot of Mt. Moon, and his newborn cub Lune grows curious about humans after finding a music box in the wreckage of an airplane. Meanwhile, the greedy treasure hunter Ham Egg leads a band of rough men through the jungle in search of the Moonlight Stones, said to hold the power to end the world's energy shortage. Caring only for profit, they wreck the land as they go, while two other humans, Dr. Moustache and Lemonade, try to stop them. When a virus spreads through the jungle and sickens many animals, Dr. Moustache arrives with a cure. Leo must fight to protect his family and home as the hunt for the stones leads up the perilous slopes of Mt. Moon.