Sudden downpours wash away the bright sunshine of years gone by. Every upheaval comes with gloomy skies and torrential rain. Disappointment is a song in youth, woven together with laughter. Family, friendship, and hazy young love shift unpredictably in the machine of time. Letter after letter, written on blue paper, is sent into the unknown, filled with words for her father and recounting her growth and the true feelings she never dared to speak aloud.
Li Huai lost her father in childhood. She seems like anyone else, yet when it comes to emotions, she stubbornly shuts herself away. She wants only to remain forever immersed in her father's love from childhood, remembering and missing the childhood friend she once liked, Jin Yang. After a turning point in her life, she meets a classmate, Wang Yuze, who gives her endless care. She accepts him as a friend, but cannot face his love for her.
Years later, when Li Huai is nearly twenty, she meets Jin Yang again and spends her days with him once more. Wang Yuze continues to stay by her side as always, and by chance, he comes across her blue letters. Secrets hidden between their parents stand in the way of the young people's relationships. University activities and encounters with the wider world set off a chain of events. There is also Lu Feishan, her closest friend at university, and Lin Ya'er, her supervisor in student affairs. These people, each with a colorful personality, experience the joys, successes, and upheavals of the passing years.
