The Tunnel to Summer, the Exit of Goodbyes - I just want to be with you even sacrificing everything in this world.

Rumors have it that there exists an Urashima Tunnel, where people can have their wishes granted, but at a price. Kaoru Touno is a high school student who lives with his father. One rainy day, while waiting for the train, he notices a girl standing beside him without an umbrella. Thinking she needs it more than he does, Kaoru lends her his umbrella. To return it, the girl suggests exchanging phone numbers, and Kaoru learns her name is Anzu.

The next day, Kaoru is surprised when the teacher introduces Anzu as the new student. Anzu mentions that it's convenient for them to go to the same school, as she no longer needs to find Kaoru to return the umbrella. However, Anzu appears aloof, and her classmates struggle to impress her. She often reads manga alone in the classroom. When a mean girl tries to tease Anzu by stealing her manga book, Anzu demands its return and ends up punching the girl.

One day, when Kaoru returns home, his father suddenly beats him and blames him for ruining the family. Kaoru decides to leave home and accidentally stumbles upon the Urashima Tunnel. Upon leaving, he discovers his sister's shoe and a supposedly deceased pet. His father is relieved that Kaoru has returned, as he had been missing for a week. Kaoru realizes that the cost of getting what he wants from the tunnel is his time. The following day, he meets Anzu at the tunnel and shares his knowledge about it. They both have something they want from the tunnel: Kaoru wants to resurrect his sister, while Anzu wants an exceptional talent for drawing manga. They decide to work together and conduct research before entering the tunnel. Through their experiments, they learn that 3 seconds inside the tunnel is equivalent to 2 hours outside. As they spend more time together, their relationship grows closer.

Anzu decides to enter the tunnel with Kaoru to retrieve her work. Although she succeeds, 3 days have passed outside the tunnel. Anzu then reveals her ultimate desire: to gain an exceptional talent for becoming a famous manga artist. She shares her draft with Kaoru, who believes she is already talented enough to achieve her dream. Kaoru's wish remains the revival of his sister, as he feels responsible for her death and hopes it will fix his family. They plan to enter the tunnel together on the anniversary of his sister's death, wondering what the world will be like when they emerge and if they will still be relevant after being gone for several years.

One day, Kaoru's father introduces a woman as his stepmother, which shocks Kaoru and leads him to question if reviving his sister is worth it. He convinces Anzu to postpone their plan and disappears. Anzu receives a message from Kaoru stating that he has entered the tunnel to search for what he desires. He advises Anzu not to join him because she already possesses the talent she seeks. Devastated, Anzu realizes that what she truly wants is Kaoru. Meanwhile, inside the tunnel, Kaoru keeps running and discards his phone after sending his final message to Anzu. He enters another world where he reunites with his sister. Eventually, he retrieves his lost phone and begins receiving texts from Anzu. Encouraged by his sister, Kaoru decides to chase after Anzu rather than staying in this world. He texts her back and races back to the tunnel's entrance. Anzu, now a successful manga artist, still carries the umbrella Kaoru lent her when they first met. They finally admit their love to each other and share a kiss.

This story reminds us to cherish the moments we have with our loved ones. It is rare to have someone waiting for us for so long. Time is precious, so let's make the most of it and not let it slip away.