Chen Laoqi has ferried passengers across the Qinghe River for forty years. After a torrential rain, the old boatman discovers a black flood dragon named Ao Chuan beneath the bridge, bound for twenty years by a dragon-subduing rope. By helping him, Chen Laoqi becomes entangled in an old case involving the Qinghe Underwater Palace and uncovers the truth behind the flood five years ago and the deaths of Ahe’s parents. As the scarlet fiend beneath the river awakens and the seven channels spiral out of control, Chen Laoqi must rely on a battered boat and a water-cleaving blade to make this crossing to the end, through floodwaters and supernatural beings.