Shen Wantang activated the "parallel time and space pen pal" power 300 days before the college entrance examination. She can communicate with herself under different choices. Each communication will change a key node in the past, but will trigger a "memory paradox" - some people's memories will be confused, and the physical rules of reality will be briefly distorted. She must choose between "sweet" (restoring friendships, improving grades) and "sour" (being misunderstood by the people she likes, losing some friends) through letters, and the final recipient of all letters is Xu Xinghe, who gave up his future for her.
