In 2147, humans have survived in underground shelters for three hundred years. After a nuclear war destroyed the surface, humanity struggled to survive in twelve underground cities. Jiang Che, 27 years old, is a hydrological engineer in Underground City No. 17, Responsible for maintaining the deepest circulating water system in the city. One day, while inspecting an abandoned pipeline, he accidentally fell into an ancient cave. There is no radiation in the cave, there is air, and there is light - coming from the dense blue veins on the walls. Those lines beat like blood vessels, and with every beat, strange images flashed through Jiang Che's mind: the burning sky, the floating city, countless kneeling figures, and a voice repeatedly said: "For the third time, they failed again."
When he woke up, he held a fist-sized crystal in his hand. The crystal is completely transparent, with a miniature galaxy suspended inside. There is a blue star in the center of the galaxy, shining brightly. After the Supreme Council of the Underground City learned about it, they immediately sent people from the Security Bureau to arrest him, saying that he was a "civilized pollution source" and must be isolated and examined. At the same time, a mysterious organization calling itself the "Gravekeeper" came to his door and told him a shocking secret: human civilization has been destroyed twice, and each time it was wiped out by some force when it reached its peak. That crystal is the "source core" left behind by the previous civilization, recording the truth of their demise. The third destruction will come in three months.
Jiang Che was forced to flee, and with the help of the tombkeeper, he gradually discovered that: the underground city is not a refuge, but a breeding ground; the Supreme Council is not a savior, but a caretaker; and the blue star flashing in the source core is the countdown of the earth.
