The explosion at the Geunyang chemical plant 19 years ago was not an industrial accident. There was a secret experiment to awaken 'it,' which had been dormant beneath the city. Dangerous particles that had slept for billions of years spread throughout the city via the factory's smokestack. The city was devastated by the disaster, but people continued to live there, and children were born. 'It,' which had been released above ground in the past, coexists in the city's darkness.

In the summer of 1999, the children born the year after the accident turned 18! Once again, the secret in the darkness is about to awaken...

In 1999, various mysterious events unfold in a village at the end of the century. Fearless girls and children investigate these incidents, encountering all sorts of unknown monsters. Meanwhile, adults attempt to cover up the incidents, and 'it,' the source of everything, reveals itself!