The school is on a trip to the beach. Unbeknownst to them, the area is infested with one-eyed jellyfish, and Riiya and Shiine get stung. The only hope to save them is an herb from down the coast, but the inlet where the herb is found is guarded by a horrible monster. Chacha, Orin, Marin, and Mr. Rascal go to the inlet to get the herb, but they are discovered by the monster, who turns out to be Umibōko, the younger sister of Umibōzu, Marin's would-be boyfriend. She is out for revenge on Marin for rejecting Umibōzu, but Marin disappeared into the cave while the monster was preoccupied with the others...