As marine biologists, Eric and Hannah have seen more of the world’s oceans than most. Whenever they head underwater in their research submarine, the Crawler, they feel at home. But home isn’t always as sweet as they say. On an ill-fated dive in the Pacific, the pair find themselves trapped in an underwater cave system, with hopes of survival decreasing by the breath, and with a desperate crew of friends on the surface above doing their best to save them.