As the Straw Hats and Ratchet's crew venture deeper into the cave, they encounter various mechanical traps and gadgets, revealing the advanced technology of Mecha Island. They also encounter Karasuke, a skilled inventor and former member of Ratchet's crew who now opposes him. Karasuke helps the Straw Hats navigate through the obstacles and offers to join forces with them to stop Ratchet.
Finally, they reach the heart of the island, where the giant turtle is sleeping. Ratchet activates his castle's control device and the turtle awakens, sending shockwaves throughout the island. The Straw Hats and Karasuke come face to face with Ratchet and his henchmen in a fierce battle, but are ultimately no match for Ratchet's powerful mecha suit.
In a desperate attempt to stop the turtle and save the island, Karasuke sacrifices himself by destroying the control device, freeing the turtle from Ratchet's control. The Straw Hats and Ratchet's crew join forces to stop the turtle from crashing into the nearby island, and they are successful in saving both islands.
In the aftermath, Ratchet and his crew are taken into custody, and the Straw Hats bid farewell to Karasuke, honoring his sacrifice and legacy as a skilled inventor and ally. The movie ends with the Straw Hats sailing off into the horizon, ready for their next adventure.