One day, while Chiikawa and Hachiware are relaxing in the plaza, Usagi suddenly arrives with a flyer slapped onto Usagi's face. Checking the flyer, Hachiware discovers it is an invitation with the words: "An Invitation to a Special Island."
Lured by the tempting words on the flyer, Chiikawa and friends decide to join the island training camp. Boarding the ship alongside Rakko, who remains suspicious of the flyer's claims, the group sets foot on the island and receives a grand, warm welcome from the Island residents.
While spending a wonderful time together, Chiikawa and friends are suddenly told a shocking truth: a massive "Siren" living on the island has been abducting the Island residents one after another. Before long, the group gets swept into the plan to subjugate the Siren. Trial after trial awaits Chiikawa and the others as the group pushes forward with earnest determination.
Can everyone overcome these hardships together...?! And what exactly is the true "secret" hidden on this island...?!