Daniel Kintner and Susan Watkins are frustrated that their hard-working lives do not allow them to spend much time together. They decide to go on a scuba-diving vacation to help improve their relationship. On their second day, they join a group scuba dive. A head count is taken and the passenger total is recorded as 20. Daniel and Susan decide to separate briefly from the group while underwater. Half an hour later, the group returns to the boat; two members of the group are inadvertently counted twice, so the dive master thinks that everyone is back on board and the boat leaves the site. However, Daniel and Susan are still underwater, unaware that the others have returned to shore. When they resurface, the boat has gone. They believe that the group will soon return to recover them.
Stranded at sea, it slowly dawns on Daniel and Susan that their boat is not coming back for them. They bicker, battle bouts of hunger and mental exhaustion and realize that they have probably drifted far from the dive site. They also realize that sharks have been circling them below the surface. Soon, jellyfish appear, stinging them both, while sharks come in close. Susan receives a small shark bite on the leg, but does not immediately realize it. Daniel goes under and discovers a small fish feeding on the exposed flesh of her bite wound. Later, a shark bites Daniel and the wound begins to bleed profusely. Susan removes her weight belt and uses it to apply pressure to Daniel's wound, but he appears to go into shock. After night falls, sharks return and attack Daniel during a storm, killing him. The next morning, Daniel and Susan's belongings are finally noticed on the boat by a crew member and he realizes that they must have been left at the dive site. A massive search for the couple begins immediately.
Susan realizes that Daniel is dead and releases him into the water, where sharks pull him down in a feeding frenzy. After putting on her mask, she looks beneath the surface and sees several large sharks now circling her. Susan looks around one last time for any sign of coming rescue. Seeing none, she removes her scuba gear and goes underwater to drown before the sharks can attack. Elsewhere, a fishing crew cut open a newly caught shark's stomach, finding a diving camera (apparently that of Daniel and Susan). One of the fishermen asks offhandedly to another, "Wonder if it works?"
Blanchard Ryan as Susan Watkins
Daniel Travis as Daniel Kintner
Saul Stein as Seth
Michael E. Williamson as Davis
Cristina Zenato as Linda
John Charles as Junior
Estelle Lau as Affected-Ear Diver