One Piece: Season 1, Episode 591

Chopper's Fury! The Master's Inhumane Experiment!

The children and the Straw Hats are in a cave with Brownbeard tied up talking about the Master. Zoro, Sanji and Brook go to look for Kin'emon, who had gone to look for his torso that Brook saw roll towards the ocean. The children start complaining and falling over from pain. Chopper, who had been testing them for any sicknesses, revealed that they had been given a highly addictive drug and were suffering from withdrawal symptoms. The children start attacking the Straw Hats, who can't fight back without injuring the children. Usopp eventually is able to put them to sleep with one of his new attacks. Back in the lab, Law gave Smoker's heart to Caesar, who had sent the Yeti Cool Brothers to deal with the Straw Hats. The Yeti Cool Brothers are then shown talking to each other while Zoro, Sanji and Brook were lying unconscious nearby.