Thirty years after a landslide sealed explorers in a massive cave system, Dr. Nicolai (MARCEL IURES) and his assistant Kathryn Jennings (LENA HEADEY) have found and reopened the cave. Yet, the fact that its entrance is filled with water means they'll need aquatic spelunkers to guide them through the filled chambers and passageways.
Accordingly, they call Jack McAllister (COLE HAUSER) and his team for the job. Along with Jack's brother Tyler (EDDIE CIBRIAN), the crew includes second in command Top Buchanan (MORRIS CHESTNUT), hotheaded Briggs (RICK RAVANELLO), expert climber Charlie (PIPER PERABO) and lead man Strode (KIERAN DARCY-SMITH). They arrive on the scene in Romania and immediately enter the cave and its flooded areas, with Strode leading the way.
When he encounters an odd creature a mile or so in and the rest of the crew then loses his video transmission, they set out to find him with documentary filmmaker Alex Kim (DANIEL DAE KIM) in tow. As they get deeper into the cave system, they soon realize -- from the human bones -- that they're not the first ones there, and that they're not alone, as it seems some previously undiscovered animal species is also down there.
Things go bad when the team realizes the creatures are carnivorous and hunting them, and then get even worse when an underground landslide closes off the exit. Three miles in, one mile down and with no one expecting them for twelve days, Jack and his team try to find another way out, all while trying to avoid the large, winged creatures that want to make a meal of all of them.