Twelve-year-old Mashiro Ogawa lives on a small island in the Seto Inland Sea. One summer day she meets a boy from Tokyo named Haruto Yuki on the beach. They spend the day together, exchange a white piece of sea glass for a blue one, share a first kiss, and promise to meet again the next morning. But Haruto never returns. Mashiro carries the memory of that single day as her first love, believing him to be her fated person.

Four years later, Mashiro moves to Tokyo to live with her grandmother and transfers into a new high school. On the first day she encounters a boy with the same name as the one from the island. Certain it is a miraculous reunion, she approaches him, but he coldly denies ever meeting her and tells her she has mistaken him for someone else. This boy is also named Haruto Yuki. He is handsome, cool, and a member of the basketball team, but he keeps all girls at a distance. The reason traces back to a lie a classmate told in middle school that caused him to lose his place on the team, leaving him with a deep mistrust of romantic involvement.

Mashiro is confused and hurt, but she cannot stop thinking about him. Despite his cold exterior, he occasionally shows unexpected kindness, and she finds herself falling for him all over again. As they spend more time together in class and through school events, Haruto begins to soften. He eventually admits the truth: he is indeed the boy from the island. He had come down with a high fever the day after their meeting and was unable to keep their promise, and he carried guilt about that for years. From that point, the story follows the two as they navigate a relationship built on a rediscovered first love.

The narrative moves through several key arcs. During the cultural festival, Mashiro and Haruto spend the day together and grow closer, but Mashiro witnesses her close friend Hime confessing to Haruto at the after-party. Hime is a pink-haired girl who had been skipping classes in the infirmary and was Mashiro's first real friend in Tokyo. Haruto gently turns Hime down, saying he likes someone he is waiting for, but Mashiro struggles with guilt and holds back her own feelings. Meanwhile, a boy named Toru, Hime's childhood friend and a known flirt, develops feelings for Mashiro and openly declares himself a rival to Haruto, creating a love triangle.

During summer break, the group visits Mashiro's home island. The distance between Mashiro and Haruto shrinks dramatically, and he tells her she was his first love. At the summer festival, under a sky full of fireworks, Mashiro confesses her current feelings to him. He accepts, and they become a couple. From here the story shifts to the challenges of a new relationship. Their first date turns into an unexpected overnight stay at Haruto's house. They take a trip together that gets complicated when a snowstorm leaves Mashiro lost and the two end up sheltering alone in a mountain cabin, deepening their emotional and physical bond.

Later, the love triangle resurfaces when Toru confesses to Mashiro, causing misunderstandings and distance between her and Haruto. They work through it and reaffirm their commitment. In the final arc, Mashiro and Haruto begin living together after Haruto writes a heartfelt letter to Mashiro's father to gain permission. At the same time, Hime, who had been quietly dealing with health issues, collapses. The story concludes with the entire group rallying around her and a resolution that emphasizes collective happiness and the strength of the bonds they have built. The series ends with Mashiro and Haruto having built a life together, surrounded by the people who matter most.