Yuto, Himari, and Rinko are invited by their class representative for a trip to the beach as a welcoming gift for Himari. While walking home from school, Yuto reluctantly agrees to take Himari swimsuit shopping, in which Himari goes ahead of him. While calling Rinko, rain starts to fall, forcing Yuto to hide under a bridge where Shizuku warns she will kill him if he goes on with his Demon Slayer heritage and vanishes with the rain clearing up. While at the beach, it is revealed that Himari has a fear of water, in which Rinko throws her an inflatable whale toy to float on. While the others are having fun, Himari, far from the shore, gets a startling visit from Yuto when a giant wave appears from behind and crashes onto them, in which both of them survive but are dragged down into the ocean by a set of tentacles. When Yuto comes to in a cave, he meets Shizuku, who warns him that his awakening as a Demon Slayer will destroy the balance between humans and ayakashi. Just when she is about to kill him, Himari intervenes but another mizuchi appears from behind and grabs her while Shizuku conjures up a whirlpool to drown her. Yuto quickly jumps in the whirlpool and manages to rescue her but drowns after. Himari saves him by stabbing her leg with one of Shizuku's ice spikes, causing the whirlpool to disintegrate from her blood and allowing her to kill the mizuchi inside of it. Himari tells Yuto to run while she holds Shizuku off despite her injury but Yuto refuses, telling her not to sacrifice herself for his sake. Deducing there is something wrong with them, Shizuku withdraws from the battle not before telling them that she will kill them next time and disappears.