The Siren arc

In a cave at night, the three meet a big black-and-white creature, the Siren, and mermaids. The Siren is furious that a fellow mermaid was eaten, and believes someone on the island is the culprit.

The next morning, the islanders confess that the pay on the flyer never existed, and ask them to exterminate the Siren, who has been taking islanders away. Rakko-sensei is taken, the friends who go to the rescue are caught too, and Chiikawa falls from a cliff and wanders the forest alone.

Chiikawa reaches Shimajiro's shop and gets their strength back by eating katsu curry. While Shimajiro distracts the Siren, the friends escape, and together they make a super-spicy curry to silence the singing voice. Singing, dancing, and tickling the Siren's mouth open, they pour the curry in.

With the power of its song sealed, the Siren sees the tears of a leaf-headed islander who stepped in as a shield, and promises not to attack anymore. That islander, who had collapsed, wakes up once the batteries in its body are replaced.

Ending

The flashback that follows shows the two leaf-headed islanders learning of a rumor about mermaid meat, suggesting they may have eaten a mermaid. On the night of the feast, Chiikawa remembers the batteries, some red grains, and something written in a book, but doesn't say anything even when Hachiware asks. With the culprit never identified on the island, the group goes home by boat, and the two move to another small island in a little boat.