Deep in the Romanian forest, a team of scientists stumbles upon the ruins of a 13th-century abbey. On further inspection, they make a startling discovery - the abbey is built over the entrance to a giant underground cave system. Local biologists believe the cave could be home to an undiscovered ecosystem, so they hire a group of American cave explorers to help them investigate its depths.

Jack (Cole Hauser) and his brother Tyler (Eddie Cibrian) are thrill-seeking professional cave explorers who run a team of the top divers in the world. The crack unit, which also includes Charlie (Piper Perabo) and Buchanan (Morris Chestnut) immediately begin their exploration. But what they find deep inside the caves is not just a new ecosystem, but an entirely new species altogether.