Fifty years after the Black Tower descended upon Blue Star, high school student Chen Xi's friend Lin Fan mysteriously disappears after a tennis match. There is no trace of him on surveillance footage, no school records, and everyone denies that Lin Fan ever existed. Only Chen Xi remembers him. Following a mysterious tennis ball that keeps vibrating, Chen Xi enters a hidden world where supernatural beings and impostors coexist, only to discover that far more than one person has been forgotten—and all the clues ultimately point to the Black Tower that has never been opened.