A group of oil company employees from Vaxxilon find themselves trapped in a dire situation while working on a boat in the Indian Ocean, near the coast of Somalia. The night shift supervisor, Karsen, sends four divers—Engel, Mitchell, Jones, and Hurst—down in a diving bell to repair a damaged oil pipeline.
After successfully completing their task, the crew begins their ascent, but their plans are abruptly disrupted when a powerful storm strikes, leading to the catastrophic failure of the cables that support their diving bell. In a desperate attempt to communicate with the surface ship, one crew member ventures out only to discover the lifeless body of Karsen and the wreckage of the support vessel on the ocean floor. Now faced with the harrowing challenge of survival, the four divers must rely on their diving skills and knowledge to escape from a staggering depth of 670 feet (200 m) while managing their dwindling oxygen supply.
In a hasty decision spurred by panic, Hurst opts to search for the ship for extra oxygen. Unfortunately, he succumbs to hypothermia, spiraling into a state of delusion and ultimately refusing to return to the safety of the bell. Realizing that Hurst faces a slow and painful death, Jones makes the agonizing choice to end Hurst's suffering by cutting off his oxygen supply. Tragically, just moments from the very cylinder he was seeking, Hurst meets his end.
While Vaxxilon abandons hope for the men, their distress signal is eventually picked up by a Chinese fishing vessel. However, communication is soon lost due to damage sustained during the storm, leaving the divers in precarious uncertainty. Fortunately, they later make contact with the navy frigate HMS Marlborough, discovering that the fishing boat relayed their SOS. Unfortunately, without knowing their precise location, rescue efforts will be delayed for hours. As they begin to run out of air, Engel undertakes a dangerous dive to retrieve additional oxygen cylinders from the pipeline's drywall chambers. In a separate attempt to aid their navigation, Mitchell makes a dive to bring their locator beacon nearer to the surface. Although he is successful, his efforts come at a cost; he suffers multiple jellyfish stings and ultimately perishes.
The HMS Marlborough finally manages to detect the diving bell but requires at least an hour to deploy rescue divers. With time running out, Engel resolves to float the bell to the surface, but it becomes lodged 170 feet (52 m) away. Faced with a grim reality, Engel makes the heartbreaking decision to use the remaining dive helmet and swim for the surface himself, determined to save Jones. In a moment of profound selflessness, he shares a poignant story about how his past actions caused a young boy’s untimely death. Although Jones manages to reach the surface, he suffers from decompression sickness, bleeding from the mouth as he is rescued by the Marlborough's team. Sadly, as Engel makes his attempt to swim to safety, he tragically drowns, leaving behind only a small chain with a heart emblem as a reminder of his sacrifice.
