In an alternate early Tang dynasty ruled by an empress, the "Directorate of Heavenly Crafts" monopolizes the path to advancement for artisans, who are classified as members of a lowly caste. The protagonist, Su Jiuzhang, transmigrates as the illegitimate son of a blacksmith in Luoyang's southern market, only to be sentenced to beheading after botching a repair to the dougong brackets of the Wu Zhou Mingtang. At the brink of death, he activates the "Lu Ban Legacy Casket": a carbon-based chip made tangible. While reforging the executioner's blade, he awakens the ability to detect metal fatigue. By repairing a broken Damascus steel saber belonging to a Persian caravan, he wins the support of foreign merchants and establishes an underground workshop.
Afterward, Su Jiuzhang develops a water-powered forging hammer, provoking a crackdown by the Directorate of Heavenly Crafts. But after improving the "Nine Dragons Sky-Piercing Cauldron," he is summoned to the palace. Meanwhile, the carbon-based chip deciphers bronze recipes and reproduces the lost chromium-salt plating technique of the Sword of Goujian, King of Yue. The empress champions artisans from humble backgrounds under the banner of "auspicious forging," countering the Directorate of Works, controlled by former Tang loyalists.
Yet Su Jiuzhang's inventions do not always go smoothly. His design for a "self-chiming palace clepsydra" draws Confucian condemnation, but he saves Luoyang's granaries by predicting a solar eclipse. He also uses saltpeter to make ice, presenting it as a manifestation of the Black Tortoise to undermine Buddhism. It turns out that the carbon-based chip was left behind by Lu Ban, the first Artisan Sage, who was himself a transmigrator. It must be unlocked by gathering fragments of the Nine Tripod Cauldrons.
When Turkic Wolf Guards attack Chang'an with Greek fire, Su Jiuzhang turns the tables with the "Three Supreme Principles of Natural Philosophy." He rises through the ranks and is enfeoffed as the Marquis of Heavenly Crafts, only to find himself trapped in the deadly "Nine Locks of Gongshu," laid by powerful clans and heirs to the Mohist tradition.
At last, he uncovers the ultimate secret of the Lu Ban Casket: the carbon-based chip is actually a seeder from an interstellar civilization. It demands that he choose between becoming an immortal mechanical sage who protects the dynasty and destroying the seeds of technology to return to mortal life. Su Jiuzhang blows up the chip's core and hides the Illustrated Treatise on Heavenly Crafts and Natural Philosophy in a merchant caravan's cargo, leaving the ending open to endless speculation. "To forge iron, you must first be strong yourself,
