"O Mar Por Testemunha" is a film about a generation´s values told in the form of a thriller. Gloria (Dominique Swain), her boyfriend Danny (Scott Bairstow), and their best friend Jeff (Henry Thomas), Americans living in Brazil, are getting ready for their last weekend together in Angra dos Reis before Danny goes back to the United States.

Andrew (José Wilker), Gloria´s father, is bankrupt. He obliges the young people to take Marcos (Sebastian De Vicente), the son of his business partner, along with them. Marcos´ father is the one who is keeping Andrew alive financially.

The four young people take a boat ride to Angra dos Reis. They have a lot of fun and drink some caipirinhas in order to relax. Danny and Jeff go diving and discuss business and they leave Marcos and Gloria alone on the boat. After talking about this and that, Marcos and Gloria get close to each other and kiss, being caught in the act by the other two Americans who are coming back to the boat.

Danny and Jeff throw Marcos overboard. They go off in the boat to take a trip around a nearby island in order to scare him. But when they come back to pick him up, he has disappeared.
From this moment on, the relationship between them starts to fall apart. Each time they return, the boat becomes more claustrophobic. Their instinct of survival leads them to think about what is necessary for them to resolve their individual problems.

In this process, the true personality of each person comes to the fore, closing the cycle of this tragedy in which well-off young people, who have everything necessary for sucess, become victims of their own selfishness.