The protagonist, "Asahina Yuya", lost his parents when he was young and was taken in by his grandparents who lived far away. He had to say goodbye to his childhood sweetheart, his older sister, with whom he was very close. What supported him through all this was the promise he made with his sister, "When I grow up, I'll marry you, Yu-kun."

Time passed, and Yuya returned to "this town" where he used to live for his studies. He met "Enomoto Chise", the school nurse, and "Shiina Mizuki", his homeroom teacher, at the school and felt a sense of familiarity from them. He started to befriend them, wondering if maybe...