A team of underwater cave divers embarks on an expedition to explore one of the least accessible cave systems in the world, the Esa-ala Caves of Papua New Guinea. The group, led by master diver Frank McGuire, includes his 17-year-old son Josh and financier Carl Hurley with his girlfriend, Victoria. Tensions run high due to strained relationships and differing personalities among the crew. As the team delves deeper into the cavern, they meticulously map the unexplored maze of subterranean tunnels and river systems, admiring the stunning, untouched environment around them.

During their expedition, a violent tropical storm hits the surface, causing the cave entrance to flood and blocking their exit route. Cut off from the outside world with dwindling supplies and low on options, the group is forced to navigate the underwater labyrinth to find an alternative way out. Along the dangerous journey, they must contend with the psychological toll of entrapment, rising water levels, and the treacherous underwater terrain that threatens their survival. Personal conflicts and panic spread as they confront the real possibility that they may never resurface.