On a stormy night, delivery driver Xiao Zhen is struck by lightning. When he wakes up, he's in Lin'an, the capital of the Southern Song, eight hundred years in the past, working as a restaurant server.
The physics and chemistry knowledge in his head becomes an untapped gold mine in this era. He improves the city's drainage, builds a business empire, invents the pedal-powered paddlewheel, and establishes assembly-line workshops. Starting with nothing, he becomes Lin'an's richest man. Then, by sheer coincidence, he rescues Princess Jin, who's traveling incognito on West Lake. The emperor appreciates his talent and summons him to court. When the Jin army invades the south, Xiao Zhen is appointed commander. Using kites for reconnaissance, sign language for communication, landmines for ambushes, and bamboo tubes to relay orders, he deploys tactics from another age that leave the Jin army trembling in fear.
But after winning the war, he faces hidden daggers at court. A treasonous chancellor plots the princess's death, and the powerful minister who succeeds him joins forces with the emperor to lay a deadly trap. At the decisive battle of Wokou, intelligence is leaked and reinforcements turn against him. With just seventeen survivors, Xiao Zhen retreats to a cliffside waterfall.
Behind the waterfall lies a valley full of peach blossoms. And the woman he thought had died beneath the cliffs is crouched by the stream, washing medicinal herbs. She looks up, and their gazes collide across hundreds of days and nights of life and death.
Hundreds of years later, an archaeological team discovers an astonishing secret carved into the walls of a cave in the valley: a map of the solar system, the structure of a steam engine, and an atomic bomb's mushroom cloud. Beneath them is a line of crooked handwriting: "Xiao Zhen was here."
A delivery driver from the modern world leaves behind a message for the world a thousand years later, a code that spans time and space.
