Harold Gern, a successful businessman from New York who has been in a lot of legal trouble recently, is spending a holiday in Puerto Rico with his attractive wife Evelyn, whom he married "between trials". They are joined by Martin Joyce, Gern’s friend and lawyer, who has come to discuss legal matters. Not in the mood to talk business, Gern invites him along on a boat trip during which all three try out some newly bought scuba diving equipment. When they resurface they realize to their astonishment that they are unable to breathe without using their oxygen tanks. They climb back into their boat and find Manuel, their servant, dead on board asphyxiated.