Diving instructor Cass gives lessons in diving off the coast of the island of Sanga in Thailand. However, an incident leads to the death of one student and a serious injury to another. Cass realizes that her dreams of a calm life on the island are in jeopardy. Her dark past becomes known, and now she herself is in danger. Sanga becomes a place of unexplained deaths, and now the main goal for everyone is to survive.