An underwater UFO destroys a ship in the Atlantic. Straker takes the best available sub crew to investigate on Skydiver. During the search, the UFO attacks and Skydiver is seriously damaged. The UFO escapes into the atmosphere, but Sky One is able to launch and destroy it. The rest of Skydiver sinks and comes to rest on an undersea ledge. The crew begins to abandon ship, however the damage has forced them to leave one at a time through a single, barely working escape hatch which requires 90 minutes to recycle. Nina Barry tries an alternate escape tube, but it fails. Two of the crew members escape to a rescue plane on the surface, and one dies of injuries. Will Straker and Barry be able to escape before the air supply runs runs out?