Summary:
After Lina reveals that she searched the entire ship, Luffy decides to destroy the ship to find Sayo. The Straw Pirates manage to defeat Doran’s men, but Doran reveals a large cannon and points it at Sayo. Sayo tells Lina that their father really cared about them and she believed that his research would bear fruit and left to protect them from her pursuers. The Straw Pirates manage to rescue Sayo and destroy Spa Island, noticing that the X on Lina’s father’s map points to an X-shaped underwater volcano opening beneath the island. A few days later, Sayo and Lina finish the gem and send it to the Straw Hat Pirates, but when sweet rain falls on Sunny, Luffy accidentally loses it.
The Straw Hats realize that Spa Island was just a hoax. Doran is revealed to be working with Foxy to get the secret ingredient for the gem. Lina searched everywhere but found no sign of Say. She begs Luffy to save her sister because she wants to see the gem.
Luffy wishes to destroy Spa Island, much to the crew’s protest. Zoro agrees to the plan. Luffy tells Franky to take care of Sunny while the crew begins destroying the island ship, causing flooding in many areas and also fighting Doran’s henchmen.
An angry Doran reveals a massive cannon, showing Sayo tied up on the roof and within range of the cannon. Doran then shows off the firepower of his cannon, leaving Sayo at the mercy of the greedy owner. He demands that the notebook be handed over to him if Doran does not execute Sayo. Desperate, Sayo insists that Lina complete their father’s research and produce a gem that would save their village.
Lina remembers how her father didn’t come home from work. Their father soon arrived to look at the cake for him. He also shows 30-year-old seashells that could prove useful in his research. He also ate the cake, but the heat from the sun set him back, so he collapsed. Their father also said that Lina was determined to throw a birthday party for him. When their father was leaving, he didn’t want greedy men to take his job.
Lina and her father once hunted for a clover leaf.
Lina then realizes that she very well forgot her four-leaf clover and refuses to hand over the notebook to Doran. Robin uses Seis Fleur to hold the greedy businessman down, then Usopp fires a Special Tobasco Star to heat up his mouth. Nami and Chopper try to save Sayo, but find that the lock needs a key. Zoro then cuts through the cannon with the Great Buddha Slayer and frees Sayo. Brook relaxes as Usopp admonishes him.
Desperate, Doran releases the ship’s ballast and tries to blast Luffy, but Luffy dodges, goes into Third Gear and slams a giant ax into the cannon, defeating the corrupt Doran and splitting Spa Island in two. Porche wakes up Foxy as the Thousand Sunny sells away. Lina then catches Luffy recounting how he forgot about the side effects of the Third Transfer. While in the water, Lina notices a large X as Sanji brings them out. The X that Rina found is an underwater volcano.
A round rainbow forms in the sky, leaving the crew in awe. They soon discover that Round-Rainbow is the jewel of the sea. Doran swims away from Foxy who is chasing them.
A few days later, Nukky arrives using bird form to deliver a report from Lina about how they made the special cake and how successful they were in their experiment. Luffy watches the gem and sweet rain falls. During the rain, Luffy loses the gem and says, “Oops, he dropped the gem.” Nami then yells at them to find it again.