Centuries later, a group of modern-day Soviet and British plunderers search for the long-lost abbey during the Cold War era. They discover the abbey is built above a vast cave system, but it is completely blocked off by an intricate floor mosaic. Trying to blast their way in, they cause a landslide that buries the abbey, trapping the men in the cavern below. They descend further into the cave in hopes of finding a way out, even as they hear strange sounds in the darkness.
Some time later, present day, a new team, led by Dr. Nicolai, with his associate Dr. Kathryn Jennings and cameraman Alex Kim explore the site, and the mythology behind the winged demons depicted in the mosaic on the abbey’s floor. Local biologists believe the cave could contain an undiscovered ecosystem, so they hire a group of American spelunkers led by brothers Jack and Tyler McAllister – thrill-seeking professional cave explorers who run a world-famous team of divers. They arrive in Romania with a modified rebreather allowing a diver to remain submerged for up to 24 hours. The diving team includes rock-climbing professional Charlie, first scout Briggs, sonar expert Strode, and survival expert Top Buchanan.
Briggs is chosen to scout ahead; when contact is lost, the group presses on in the likelihood it is simply an equipment malfunction. After the group finds Briggs safely downriver, Strode is suddenly attacked and dragged away by a large, unknown creature. His water scooter explodes and causes a cave-in, forcing them to follow the river and search for a new way out. Jennings and Nicolai discover a strange parasite in all of the lifeforms they find. Unlike the known cave species, which have adapted over generations to life underground, Jennings believes this new parasite originated in the cave environment and has never been exposed to the outside world. The team stumble across the equipment and remains of previous explorers, unaware they are being stalked by the creatures.