While on a dig in a Romanian forest, a group of scientists make a startling find. Dating back to the 13th century, they find a huge abbey, which was built over the entrance to a massive network of subterranean caves. Eager to learn what lies within the caves, the scientists discover to their peril that a large part of the cave system is under water. The scientists hire a team of American spelunkers who are trained in exploring underwater caves. The team is led by brothers Jack and Tyler. Armed with new high-tech SCUBA gear, Jack, Tyler, and the rest of their crew dive in to investigate the caves. A mishap traps them in a cavern beneath the surface, and they soon discover they are not alone.