The Tunnel to Summer, the Exit of Goodbyes

夏へのトンネル、さよならの出口

Kaoru Touno has spent five years blaming himself for his younger sister Karen's death by the time he hears of the Urashima Tunnel, a passage rumored to grant a wish in exchange for years of the wisher's own life. Classmate Anzu Hanashiro, aloof and newly transferred to his rural high school, tracks him to the tunnel's entrance and proposes they investigate it together over the summer, though her real reason for helping stays hidden. This light novel is the original source for the 2022 anime film and its manga adaptation, Ultramarine.