Sanctum follows the story of a team of underwater cave divers, on an expedition to the most beautiful, unexplored and least accessible cave system in the world. We meet Frank McGuire (Richard Roxburgh), a single-minded master explorer, his 17-year-old son Josh (Rhys Wakefield), participating under protest, and billionaire Carl Hurley (Ioan Gruffudd) who is underwriting the expedition. When a tropical storm arrives suddenly, their only exit is cut off in a flash flood and they are forced to move forward deeper into the cave. With ever-dwindling lights and supplies, they must navigate the treacherous terrain and raging water, in search of an unknown escape route to the sea. But the cave is unforgiving of mistakes and soon they are each confronted with the unavoidable question - will any of them make it out alive?