Thirty years ago, in the Carpathians Mountains of Romania, a group of explorers blows an entrance to a cave hidden under a church, but the explosion causes a landslide and they become trapped inside. In the present days, the expedition leaded by Dr. Nicolai and his assistant, Dr. Kathryn are exploring the place and they find the access through an underwater river, requesting the experienced cave divers team leaded by Jack. Dr. Nicolai discovers that, in accordance with the local legend, the church was built to seal the cave as a display of God's protective power, and that Templar Knights entered the cave to fight winged demons. When the group reaches a cave though a tunnel one mile below and three miles in, a creature attacks one of the members and his breathing apparatus explodes, collapsing the tunnel and trapping the group in the cave. Sooner they realize that they are the rescue team and they have to find an exit to survive from the attack of the monsters.