Andrzej and Krystyna are a married couple driving to the marina to spend a Sunday sailing. On their way, a hitchhiking student causes them to veer off the road. Andrzej confronts the lad, who only wants a ride. They give him a lift and invite him onboard.
Soon a more sinister tone starts to creep in and the youth is carrying a large switchblade. Andrzej torments the young man, he even tosses the boy's beloved knife overboard. After a confrontation, the youth accidentally falls overboard and disappears. Krystyna reacts badly, claiming her husband is a killer.
She goes off looking for the boy, but cannot find him while Andrzej swims to shore for help. But the young man may not be dead….