Kaoru Touno, initially skeptical of the urban legend surrounding the wish-granting Urashima Tunnel, finds himself drawn to its mysterious allure when faced with the chance to bring back his deceased sister, Karen. Despite the ominous tales of a lifespan cost, Kaoru ventures into the dark tunnel, only to unravel unexpected truths that defy the rumors. In his exploration, Kaoru is unknowingly accompanied by his enigmatic classmate, Anzu Hanashiro, who expresses a keen interest in the tunnel's secrets and proposes an alliance. As they delve deeper into the tunnel's mysteries, Kaoru grapples with uncertainty about Anzu's motives but agrees to collaborate with her throughout the summer, united by the shared goal of resurrecting his lost sister.