Kaoru Touno stumbles upon the legendary Urashima Tunnel, rumored to transcend the constraints of time and space. Whispers suggest that one's deepest desires await on the other side, at the cost of precious years of life. Intrigued, Kaoru contemplates his long-lost sister, Karen, and the tragic accident that separated them years ago. Unexpectedly joined by the enigmatic transfer student Anzu Hanashiro, Kaoru delves into the tunnel's mysteries under her guidance. Yet, as they unravel its secrets, Kaoru must confront the looming question: What does Anzu seek in return, and what sacrifices will the tunnel demand from him in the end?