"The Tunnel to Summer, the Exit of Goodbye: Ultramarine" is a manga adaptation of the light novel by Mei Hachimoku, illustrated by Koudon. The story centers around Kaoru Touno, a high school student who becomes entangled in a mysterious urban legend involving a tunnel said to grant wishes at a cost. After the death of his younger sister, Kaoru’s life is marked by grief and strained family relationships. One day, he discovers the tunnel, known as the Urashima Tunnel, which is rumored to distort time and fulfill desires for those who enter. However, the tunnel demands a price: for every day spent inside, years pass in the outside world.

Kaoru’s exploration of the tunnel leads him to cross paths with Anzu Hanaki, a classmate with her own reasons for seeking the tunnel’s power. Anzu is determined to use the tunnel to change her past, but her motivations are shrouded in secrecy. As the two form an uneasy alliance, they navigate the tunnel’s surreal and dangerous environment, uncovering its rules and the consequences of their actions. The narrative delves into themes of loss, regret, and the human desire to alter the past, as Kaoru and Anzu confront their personal struggles and the moral dilemmas posed by the tunnel’s power.

The story progresses as the characters grapple with the emotional and temporal toll of their choices. Kaoru’s initial goal to undo his sister’s death becomes complicated by his growing bond with Anzu and the realization that altering the past may not bring the closure he seeks. The tunnel’s enigmatic nature and the passage of time create a sense of urgency and tension, driving the characters to confront their deepest fears and desires. The manga balances moments of introspection with a sense of mystery and adventure, as Kaoru and Anzu’s journey through the tunnel becomes a metaphor for their emotional growth and acceptance of life’s impermanence.