Inside a submarine continuing its exploration in the pitch-black depths of the ocean, the protagonist falls into a deep sleep, suffering from an unknown high fever. He did not even hear the warning sounds and screams echoing through the vessel.

When he woke up late and opened the door to step out, he found only gruesome bloodstains stretching down the narrow corridor, and the other crew members were nowhere to be found. Just as he was searching the interior of the submarine, avoiding unidentified sounds coming from the darkness, a signal from a rescue submarine was picked up on the sonar screen like a single beam of light.

However, the relief is short-lived, as they realize that another unidentified threat is slowly approaching at the edge of Sona.