On an oil rig platform, located south of Jeju Island, Hae-Joon (Ha Ji-Won) works as the equipment manager for a small, but dedicated crew. Joining the crew is Jung-Man (Ahn Sung-Ki), a former colleague of Hae-Joonâ™s late father, and now newly assigned captain of the platform's underwater exploratory ship "Eclipse." Though his job is to oversee the withdrawal of the platform, Jung-Man suggests conducting one last drilling.
After 3 weeks of preparation, Hae-Joon and another crew member dive underwater to work on part of the platform. While performing their work, her partner sinks into the abyss due to an equipment failure. When Hae-Joon gets back on the platform more crew members start to die off in mysterious ways. What they soon learn is that an underwater monster has come...