Nami is sick, but the crew doesn't have a doctor to help her.

The Straw Hat crew manages to gain the trust of the people that attacked them, and are taken to the village, except for Zoro and Carue, who guard the ship. Luffy and Sanji must take Nami to the island's sole doctor who lives on a mountain-top. Vivi and Usopp discover the past of the island, and about the pirate Wapol from Dalton.

While Sanji and Luffy fight off the wilderness, the islander's worst nightmare turns true, as Wapol returns to the island. The doctor is discovered to be on the other side of the island, and Usopp, Vivi and Dalton rush to find her. Dalton however returns as he hears of Wapol's return.

Dalton faces Wapol, but only gets himself hit by arrows thrown by his former comrade Chess. An avalanche stops the battle, but injures Sanji. Luffy must carry both Nami and Sanji to the doctor, and begins his ascent.

The Lapahn help Luffy escape from Wapol, giving him enough time to climb to the castle where doctor Kureha lives. Vivi, Usopp and Zoro deal with the king's army in order to save Dalton. The recovering Nami meets a strange creature.

Nami asks Chopper to join the crew, but he isn't interested. Kureha tells Nami some of what was Chopper's background. After realizing the abilities that Chopper has, Luffy is eager to make him join the crew.

Doctorine continues to tell how Chopper lived his life and how the quack doctor Hiruluk took care of him.

More of Chopper's past is revealed, including the fate of doctor Hiruluk and Wapol's cruelties as the king. Meanwhile, Wapol reaches the castle where the crew is residing.

Wapol is determined to bring down Hiruluk's flag, and to do that, he engages Sanji, Luffy and Chopper in a fight.

Chopper demonstrates the abilities of Rumble Balls when fighting Chessmarimo. Dalton is uncovered from the avalanche.

After trying every trick in the castle, Wapol is finally hunted down by Luffy. The villagers along with Zoro, Vivi and Usopp start their ascent to the castle through a ropeway.

After finally been convinced to join the team, Chopper announces his intentions to Dr. Kureha, who doesn't take it too well; but after escaping from the castle, she has decided to give the whole Straw Hat crew a farewell gift.

Chopper joins the Straw Hat crew as they continue their journey to Vivi's homeland.