Yui Hiiragi, a university student, marks her 20th birthday with a winter journey alongside her mother to an astronomical observatory in Hokkaido. The ascent along snow-covered roads toward the summit stirs childhood memories. Throughout her youth, Yui visited this same observatory annually with her late father on her birthdays, forging a tradition tied to the location. This snowy climb on her 20th birthday acts as a catalyst, vividly recalling memories of her father.