The story is set in a fictional Japanese city near Kasaki Station, where an urban legend speaks of the "
Urashima Tunnel", a mysterious passage said to grant wishes—though at a cost. Kaoru Touno, a reclusive high school student in a rural town, commutes daily on the Oosara-Sugimori train line. One rainy evening, he meets Anzu Hanashiro, a drenched girl at the station. When Kaoru offers her an umbrella, she reveals she has no parents, to which he responds with an indifferent "good". They exchange phone numbers before parting ways. The next day, Anzu transfers into Kaoru's class from Tokyo. Despite the class's attempts to welcome her, her aloof demeanor alienates them, provoking Koharu Kawasaki, a classmate who annoys and bullies her. Anzu retaliates by punching Koharu and shocking everyone. That night, Kaoru's drunken father blames him for his sister Karen's death, and demanding him to revive her, even at the cost of his life. Distraught, Kaoru flees and stumbles upon the Urashima Tunnel. Inside, he retrieves one of Karen's slippers and his deceased pet parrot, only to emerge and discover a week has passed outside. Kaoru re-enters the tunnel the next day, encountering Anzu. They agree to collaborate to fulfill their respective wishes. Over the following weeks, their bond deepens. They learn that time inside the tunnel flows differently—three seconds within equate to two hours outside. During a visit to an aquarium, Anzu confesses her fear of the tunnel, while Kaoru reveals his wish: his sister Karen died falling from a tree, after failing to catch a
rhinoceros beetle and reconcile with him, following an argument. He wishes to undo the past. During another expedition into the tunnel, Kaoru and Anzu aim to spend precisely 108 seconds inside—equivalent to three days outside—but they exceed this time, emerging at 4 a.m. They retreat to Anzu's apartment, where she explains her own wish: her grandfather, a struggling
manga artist, became a burden on her family, and when she expressed her own ambition to follow in his footsteps, her parents disowned her and her grandfather. She desires the talent to become a renowned manga artist. The papers they retrieved from the tunnel are an old manga she wrote as a child, discarded by her father. Kaoru reads it and insists she already possesses talent. After a summer festival date, Kaoru returns home to find his father with a new partner. The next day, he persuades Anzu to delay their final tunnel trip, but soon afterward, he vanishes. Anzu, fearing the worst, rushes to the tunnel and receives a message from Kaoru revealing the tunnel's true nature: it cannot grant wishes, only restore what was lost. He urges her to pursue her dream as a manga artist so he may one day see her work when he escapes. Heartbroken, Anzu admits she simply wanted to remain with him. Inside the tunnel, Kaoru finds himself reunited with Karen in a fabricated reality. However, he begins receiving messages from the outside—an impossibility—and Karen assures him she will be content as long as he is happy with someone he loves. Realizing his true desire is to be with Anzu, Kaoru accepts Karen's death and resolves to leave. Eight years pass outside. Anzu has become a moderately successful but disillusioned manga artist. One day, she returns to the train station where she first met Kaoru and collapses in tears, confessing she has remembered him. Anzu receives a message from Kaoru and rushes to the tunnel, believing he has emerged. Inside, Kaoru slips and falls, only to awaken to Anzu's presence. They reconcile, before leaving the tunnel—13 years and 102 days after first entering. ==Characters==