The Straw Hat Pirates set sail for Crown Island, a mysterious location marked on a treasure map that Nami has acquired. As the Going Merry approaches, a ring of powerful geysers erupts from the sea, launching the ship into the air and separating the crew. Tony Tony Chopper, the crew’s reindeer doctor who has been struggling with feelings of fear and self-doubt after being teased by Usopp, is thrown from the ship and lands deep within the island’s jungles.

Chopper soon discovers that Crown Island is inhabited by a society of bizarre, talking animals. These animals, led by a young human boy named Momambi, believe an ancient prophecy: a new king will fall from the heavens when the previous one passes away. Having just lost their king, the Kirin-Lion, the animals immediately proclaim Chopper as their new ruler. Momambi, whose father was killed by pirates years ago, is fiercely protective of the island and adamantly opposes Chopper's wish to leave, insisting that he embrace his role as the King of Beasts.

Meanwhile, the rest of the Straw Hat crew lands on the opposite shore. They meet an elderly parrot who explains the island’s nature and quickly begin searching for their lost friend. Their mission becomes urgent when they learn of a threat to the island’s inhabitants. A trio of human hunters known as the Horn Eaters is terrorizing the animals. Their leader, Count Butler, is a refined but cruel violinist who, along with his henchmen President Snake and General Hotdog, seeks to consume the island’s legendary treasure—a set of magical horns said to grant immense power. Butler uses his violin to hypnotize powerful beasts into stampeding and killing the horned animals for his harvest.

As the narrative unfolds, Chopper is torn between the adoration and responsibility of being a king and his loyalty to his pirate crew. He must find the courage to protect his new animal subjects from Butler's assault while Momambi must learn to overcome his hatred of pirates and trust Chopper’s friends. The story builds to a confrontation where the Straw Hats battle Butler’s forces across the island’s unique geography, culminating in Chopper’s personal decision about whether to remain on Crown Island or continue his journey on the Grand Line.