One day, as Chiikawa and Hachiware are relaxing in a plaza, Usagi suddenly appears and sticks a flyer onto his face.
When Hachiware checks it, the flyer turns out to be an invitation that reads: “You are invited to a special island.”
Tempted by the enticing promises written on it, Chiikawa and the others decide to attend a group training trip on the island.
Along with Rakko, who is suspicious of the flyer’s contents, they board a ship and arrive at the island—where they are warmly welcomed by the island’s residents.
At first, Chiikawa and the others enjoy their carefree time there.
However, they soon learn a disturbing truth: a giant “Siren” living on the island has been abducting the residents one after another.
Before long, they become entangled in a plan to defeat the Siren.
Trials begin to appear one after another before the determined Chiikawa group—can they overcome them together?
And what is the island’s hidden “secret”…!?