One morning, a modern-day man known only as “the Fatty” wakes up in the wilderness of ancient China, separated from his only son, Su Yunfan, after a strange accident inside a cave. The familiar yet unfamiliar scenery of Tongguan confirms his suspicion: he has been thrown into the past, all the way to the Sui Dynasty, in the fourth year of Renshou. Armed only with survival instinct and deep anxiety for his son, the Fatty realizes that his only chance lies in obtaining identity and influence. With no proper clothing, money, or ancient residency permit, he must act fast. Along the way, he encounters figures who will one day shape history—Li Yuan, Fang Xuanling, and Du Ruhui. Ultimately, he is rescued by the entourage of Yang Jian, Prince of Yuzhang, the second son of Yang Guang. To secure a place in the political circle, the Fatty boldly claims to be a master of “calculation” (divination) and proves his genius by solving an ancient puzzle using modern Pythagorean logic. This is the tale of a fat man who must bend history with wit and tricks, all to ignite a beacon in the past so his missing son can find his way back.