Trade a Life for a City

The poor physician's apprentice Shen An can see "qi": those shrouded in black qi will die within three days, while those surrounded by white qi mean an epidemic is on its way. But he can only warn of each illness once in a lifetime, and every time he intervenes, he loses a piece of his lifespan, with no way to get it back. When the qi across the entire city knots into a single mass, he finally realizes that what he sees also includes an old affair from thirty years ago.