As Paul Sturges navigates the barren shores of Baja California with his family—including his wife Ines Sturges and their two young children, Audrey Sturges and Tommy Sturges—they are greeted by a town in disrepair. The locals harbor bitter resentment towards outsiders, a feeling that escalates when Paul's role as an inspector for Nixon Oil comes to light. His intentions to inspect the run-down offshore oil rig, named El Diamante, only deepens the community's animosity.
Upon reaching the rig, now the site of a foreboding oil spill, Paul encounters the last two workers, Junior (portrayed by Jorge A. Jimenez) and Chato, along with their small dog. The atmosphere is charged and perilous as Paul's family approaches in a separate boat, oblivious to the lurking dangers ahead. As they near the rig, distress flares light up the sky, signaling a deadly threat: a colossal megalodon shark, referred to as El Demonio Negro, which has struck terror into the waters.
Escaping from the rig, the family witnesses their boat's destruction and the tragic end of its captain. Chato shares an alarming story with Paul, revealing that only a few fortunate souls have escaped the wrath of the Black Demon while also disclosing the rig's broken communications, making it impossible to call for help. It becomes clear that Nixon Oil was aware of the spill yet opted to overlook it, prioritizing profits over the safety of the crew and their families.
Ines discovers the derelict safety reports for the rig and is appalled by the overt negligence that Paul, in his role, had initially approved. Confronted with his own shortcomings, Paul admits his hands were tied; maintaining his position and financial stability came at the cost of compromising his ethics. Nixon Oil's self-serving inspections fostered hazardous conditions, threatening Paul with dire consequences if he dared to challenge their authority.
Burdened by his decisions, Paul finds himself ensnared by Nixon Oil's web of deceit, forfeiting his moral integrity for economic survival, while the local populace grapples with the repercussions of the corporation's avarice and carelessness, left to mend their fragmented lives.
As Chato and Junior undertake a treacherous underwater mission to restore power to the rig, Chato becomes entranced by the ominous presence of the Black Demon—a portent he believes symbolizes Tlaloc’s, the rain god, retribution for those who exploit nature without remorse. Paul's intuition is stirred when he uncovers a demolition bomb cunningly hidden beneath the rig's structure. This shocking discovery reveals Nixon Oil's intention to eliminate Paul and obscure their role in the spill.
Faced with a life-or-death decision, Paul dives into the depths of the ocean, with his family and Chato making their escape, to seal the oil leak and confront the Black Demon. He manages to successfully stop the leak but finds himself trapped within the rig as the sinister creature closes in. As Paul straps the bomb to himself, a heavy realization dawns upon him: the only way to eradicate the Black Demon and restore balance is through his own sacrifice. With a somber heart, he delivers his final messages to his family over the radio before succumbing to the Black Demon's ominous grasp. The bomb detonates, annihilating the creature and causing the rig to collapse in a catastrophic cascade.
As the aftermath begins to settle, Paul's family is left to carry out his last wish by bringing the damning evidence to the townspeople, unveiling Nixon Oil's responsibility for the environmental crisis. As they head back to shore, a fishing boat emerges on the horizon, its timing conspicuously aligned with an approaching storm—a poignant echo that nature ultimately demands its price.
