Amid a summer downpour beside Baotu Spring, delivery worker Lin Shen rescues Su Tang, who has leukemia. She records the four seasons of Quancheng in her sketchbook, but as chemotherapy takes its toll, she gradually withers like the sunflowers on her pages. Between the aroma of youxuan pastries and the strains of Lyu opera, the two weave together the tender words of youth, until Su Tang's worsening illness becomes a black thread winding around her life. When Lin Shen discovers a gene sequence in the bone marrow registry that perfectly matches hers, the quantum clock tower atop Qingfeng Mountain reflects an unfinished experiment from fifteen years earlier. With a sketchbook, fluorescent medicine vials, and a starlight pendant, they write a love letter that crosses the divide between life and death in the rift in time. Even if they become stardust scattered across the spring waters, their love will echo forever in the morning light shimmering over Baotu Spring.
