In the pouring rain at Kasaki Station, two worlds collide when Kaoru Touno, a shy high school student from a quiet rural area of Japan, meets Anzu Hanashiro, an enigmatic transfer student from Tokyo whose life seems to be an empty canvas. As Kaoru shelters her from the rain with his umbrella, Anzu candidly shares the absence of her parents in her life, prompting Kaoru to respond that it's "good"—a seemingly innocuous comment that foreshadows the tangled complexities ahead. Their connection deepens as they exchange phone numbers, marking the beginning of their relationship.
The following day, Anzu’s arrival at Kaoru's school is met with mixed emotions. Despite the class’s attempts to welcome her, Anzu's frosty attitude alienates her from her peers, particularly the school’s mean girl, Koharu Kawasaki, who mocks her by stealing her manga and showcasing it in front of everyone. Anzu's retaliation is swift and decisive—a punch that not only stuns Koharu but also solidifies her own resistance. Meanwhile, Kaoru faces turmoil at home, as his father’s rage boils over, blaming him for the perceived abandonment of their family and unfairly holding him accountable for the loss of his sister, Karen. In a moment of desperation, Kaoru escapes the household only to stumble upon the legendary Urashima Tunnel.
Venturing back into the tunnel, carrying one of Karen’s slippers alongside his late parrot, Kaoru discovers that time behaves differently inside; a mere hour in the tunnel translates to an entire week outside. Upon returning, he finds Anzu waiting for him, and together, they forge a pact to chase their innermost desires—certain that the tunnel can help them achieve what they long for. Each visit enhances their connection, with Anzu confessing her fears about the tunnel while Kaoru reveals the heavy wish burdening his heart: to bring back his sister, whose tragic fate haunts him.
As time elapses, Kaoru and Anzu explore the tunnel's peculiarities. Their quest soon turns urgent as they aim to navigate back within a specific timeframe after collecting a series of mysterious papers. However, they miscalculate their timing and end up spending more than two crucial minutes inside. As dawn breaks, they find solace at Anzu's modest home, where she reveals her crushed ambition to become a renowned manga artist, a dream dashed by her parents’ harsh reality. The papers they found reveal a forgotten manga created by a young Anzu, cast aside by her father without a second thought, hinting at her latent potential that Kaoru becomes determined to unveil.
After a delightful date during a summer festival, Kaoru returns home only to discover his father has introduced a new presence into their lives—his stepmother. The next day, he urges Anzu to delay their last tunnel expedition, yet his abrupt disappearance plunges her into anxiety. As she rushes back to the tunnel, she discovers Kaoru’s poignant message: the tunnel offers no wishes but instead serves to address what is lost. He encourages Anzu to embrace her manga dreams, hinting that he must reconcile with his secluded existence. Heartbroken, Anzu confesses that her true desire has always been to be with Kaoru.
Inside the tunnel, Kaoru is enveloped in a surreal experience where he reunites with his sister on the day after her passing. Her joy at their reunion is bittersweet, and cryptic messages begin to reverberate from outside, presenting an impossible twist in their story. She reassures him that her happiness hinges on him finding love, leading Kaoru to a significant revelation—his heart yearns for Anzu. He resolves to accept his feelings and move beyond the shadows of his sister's death.
Fast forward eight years since their serendipitous meeting. Anzu is now a moderately successful manga artist, but her passion has dimmed, shadowed by feelings of bitterness and fatigue. Standing at the same train station where their journey began, she succumbs to nostalgia over Kaoru and her failed attempt in the tunnel, breaking down in sorrow and admitting she constantly thinks of him. Just then, a message from Kaoru arrives, hinting at his spiritual emergence from his own stasis; brimming with hope, she embarks on a journey to reunite with him.
In a twist of fate, as Kaoru loses his footing and plunges back into the tunnel, time appears to halt. Surrounded by darkness, his consciousness flickers until it is reignited by Anzu's warm, reassuring presence. In a poetic moment that transcends time, their lips meet in a gentle kiss, intertwining their fates. Emerging from the depths of the tunnel, Kaoru steps into the light 13 years and 102 days after his initial descent, forever altered by the serendipitous encounter that reshaped his reality.
