As part of a research project, Mike Nelson attempts to spend three days underwater at a depth of 250 feet in a pressure chamber dubbed Scubasphere. A tropical storm pops up, and both the cable and telephone connections attached to the Coast Guard ship above are severed. To make matters worse, the ship heads for safer waters without realizing that Mike has lost his connection to them.
Mike is invited to join a longtime friend and her workaholic husband on their expedition to explore an Aztec ceremonial pool on an uninhabited island located 15 miles off the coast of Mexico. Mike's friend unwittingly puts her husband's life in jeopardy, when she deliberately strands the trio on the island.