Based on a series of Chiikawa comics posted on X by Nagano from March to November 2023, known as the "Siren Arc". 
One day, while Chiikawa and Hachiware were relaxing in the square, Usagi suddenly appeared with a flyer stuck to its face. When Hachiware checked the flyer, it was an invitation reading “Special Invitation to a Special Island.” 

Lured by promises like “Get 100x rewards for simple island quests!” and “Limited-edition island ramen, exclusive sweets—sweet and spicy treats all practically free!”, Chiikawa and the others decided to go on the island retreat. Boarding the ship alongside Rakko, who was suspicious of the flyer's contents, the group landed on the island... only to find something waiting for them there...!? 
(Source: Official Site, translated)

Chiikawa: Ningyo no Shima no Himitsu