Deep in the Romanian forest, a team of scientists discovers a 13th-century abbey built atop the entrance to a giant underground cave system. To plumb its depths and its secrets, they hire a team of American cave-explorers led by Jack (Cole Hauser) and his brother Tyler (Eddie Cibrian).

The crack unit, which also includes Charlie (Piper Perabo) and Buchanan (Morris Chestnut), arrives in Romania with all the latest equipment and immediately begins its survey. But in the cave's darkest reaches they learn that there are some places on Earth where mankind isn't meant to go.