Truck drivers Li Mei and Zhou Erqiang, a married couple, take a truckload of watermelons into a mysterious tunnel and find themselves, along with their vehicle, in Pingyangdu, an isolated fortress on the northern frontier a thousand years in the past. After years of severe drought, the border town's water sources have dried up, leaving 500,000 soldiers and civilians on the brink of death. The couple makes a promise: as long as the tunnel remains open, they will supply Pingyangdu with water and food. Bottled water, sweet potato seedlings, well-drilling teams, and military medical facilities—their thirteen-meter freight truck travels back and forth between two eras, repeatedly bringing hope into the besieged city. Through severe drought, famine, and siege, the couple stands shoulder to shoulder with the defending soldiers and civilians, delivering truckload after truckload of supplies to forge a lifeline spanning a thousand years.