1944. When a Nazi special weapons compound comes under attack, several civilian workers board one of Hitler's experimental U-boats and embark on a daring escape. Present day: In search of a group of missing college students, Jimmy and Barbara Ciba, along with their closest friends, enter a huge, unexplored cavern system. Bill Stevens, an undercover detective, and Ranger Joannie Logan become involved in the search. Intent on killing everyone inside the cavern system, a gang of murderous drug smugglers who have been using the entrance cave to store their marijuana shipments go in pursuit. Then, the cave creatures appear. Are they intelligent, alien, friendly, of cannibalistic?