Paul Sturges, an inspector for the Nixon Oil Company, is sent on vacation with his wife Ines and their two children, Audrey and Tommy, to a small Mexican hamlet in Baja California to check on an offshore oil rig called El Diamante. The family arrives to find the town in ruins. The remaining residents of the village are wary and unpleasant, and when they discover that Paul works for the organization that commissioned the oil rig, their dread turns to rage. Paul takes a boat to the rig to see if it is worth reviving or decommissioning.
When he gets there, he discovers that Chato and Junior, two remaining rig workers, and their pet chihuahua are the only crew members left on the unsafe, outdated rig that is surrounded by an oil spill. Paul’s family ultimately decides to follow in another boat when Ines bribes the boat’s owner to carry them to the rig after they are repeatedly bothered and hounded by the irate locals. A 60 foot (18 m), 45.3 t (100,000 lb), megalodon shark known as the “Black Demon” (Spanish: El Demonio Negro), patrolling the waters, starts chasing the boat as Chato and Junior start to panic and burn flares.