Did you ever feel like this when you were a teenager: life is like carrying a heavy shell on your back and walking forward in the mud. You want to move forward but are pressed in place by the heavy shell. The harder you struggle, the faster you sink. Giving up the struggle and staying where you are only slows down the speed of sinking. It seems that no matter what we do, all that awaits us is to feel the silt submerging our bodies bit by bit, including our legs, stomach, chest, and neck, until we suffocate and die. For Luo Shu, life is something that is not worth looking forward to. Luo Shu is like a plant growing in the shade, longing for the sun but holding back because of the heat of the sun, until she met her boy. Her boy appeared in front of her on white clouds, spreading light and heat into her life, making her understand that the sun is so warm. The only pity is that her boy does not belong to her. She wrote the secret love of her teenage years into a love letter to a boy, but it was never sent to the boy, and was never opened by the boy. The boy did not know what a moving and beautiful love letter it was. However, the years went round and round, and this love letter finally reached the hands of the young man many years later...