In the distant future, an event called the "
Quiet Rapture" causes all stars, planets, and the human population to mysteriously vanish, with only those on space stations and spaceships remaining. Simon, a convict imprisoned for his involvement in destroying a space station, is forced to pilot the
SM-13 (nicknamed the "Iron Lung"), a submarine whose porthole is sealed. In order for Simon to see, he must take still images using a crude camera, and he receives limited communication from his crew on the surface. In exchange for completing the mission, Simon is promised freedom. During the dive, Simon captures images of a skeleton on the ocean floor and is briefly brought to the surface, but is tasked with collecting a sample from the skeleton. Simon demands to be freed, but the crew's commander berates him, explaining that the camera is an
x-ray imaging device, which has heavily
irradiated the crew outside of the submarine. Simon learns that he is not the first pilot of the Iron Lung after discovering a seed medallion left by the previous pilot; a fellow convict from Eden according to audio logs from the submarine's on-board computer. He captures an image of a living creature, and is again tasked to take a sample of the creature. As Simon explores the moon, blood and unidentified liquid drip into the submarine. After collecting a sample, Simon is knocked unconscious. He awakens, and the camera continues capturing images of a large monster nearby. He repairs the sub and attempts to return to a charted area, but continues hallucinating and loses touch with reality. He later finds the wrecked
SM-8, but is unable to access the data within its
black box. A woman talks to him through the radio about a godly light beneath the ocean, which Simon believes he is hallucinating. The monster attempts to push Simon into the light, which seemingly cuts through it and the Iron Lung. On the other side of the light, Simon breaks the surface to see the monster slinking back beneath the blood, and a giant eye filling a stormy red sky. Simon awakes again, and the commander says he has been missing for several days, far longer than his oxygen supply should have lasted. She tells him to recover the data of the other submarine, promising to rescue him herself if he complies, as well as revealing her name to be Ava. Meanwhile, blood and congealed organic matter collect inside the submarine. Simon discovers that the skin on his arms has become cracked and blistered due to physical contact with the blood. Simon reaches
SM-8 and accesses its data, including audio logs confirming that the blood ocean comprises human blood, and that one of the SM-8's crew began obsessively drinking it before their sub was destroyed. As the submarine becomes flooded with blood and fleshy matter, Simon learns Ava cannot rescue him. She apologizes and asks him to protect the
SM-8 data because it may save what is left of humanity. Simultaneously, the mysterious woman's voice tells Simon to destroy the data so others will not learn about the light. Shortly after, it's revealed that the mysterious woman's voice belongs to the monster that has been stalking Simon throughout the ocean. The monster destroys Ava's vessel and attempts to kill Simon next. Simon, struggling to move through the blood and fleshy material growing in the submarine and onto his body, ties the black box to a life vest as his body begins rapidly mutating. Just as the blood makes contact with the cracked medallion Simon holds, the seed within mutates into a fully grown tree that implodes the submarine from within upon being bitten by the monster, blowing open its jaw and killing both of them. The life vest and the black box are shown floating on the surface of the ocean, illuminated by a flashlight. ==Cast==