Plot
In the distant future, an event called the "Quiet Rapture" causes all stars, inhabited planets, and the human population to mysteriously vanish. Only lifeless planets floating in the void, and the few humans who were on space stations and spaceships during the rapture, remain.
A man named Simon is convicted and imprisoned for his involvement in destroying a space station known as Filament Station. He is sent to a newly discovered moon and forced to pilot the SM-13 (a submarine with a sealed porthole nicknamed the "Iron Lung") through the depths of the moon's ocean of blood. In order for Simon to see, he must take still images using a crude camera, and he receives limited communication from the crew on the surface via a radio. In exchange for completing the mission, Simon is promised freedom. During the dive, Simon captures images of a large skeleton on the ocean floor and is briefly brought to the surface before being tasked with collecting a sample of the skeleton. Simon demands to be freed and activates the camera, 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 and a voice recording left by the previous pilot. After collecting a sample, Simon captures an image of a living creature that attacks the sub, and he is knocked unconscious. Awakening in an uncharted cave system with communications lost, Simon repairs the sub and attempts to return to a charted area, capturing images of the living sea monster stalking him nearby. As Simon explores the moon he begins to hallucinate as blood slowly leaks into the submarine.
Simon finds another wrecked submarine, the SM-8, but is unable to access the data within its black box. He begins hearing a woman's voice, seemingly another convict talking through the radio, which Simon believes he is hallucinating. The voice directs Simon to a godly "light" beneath the ocean, supposedly linked to the Quiet Rapture. Upon arriving at the light, Simon experiences a vision of the blood ocean's surface, where he makes contact with both the sea monster, speaking via the woman's voice, and a mysterious entity watching him through a giant eye in a red sky. Simon awakens with the sub's communication restored, and learns he has been missing for several days, far longer than his oxygen supply should have lasted. The commander tells him to recover the data of the SM-8, promising to rescue him herself if he complies, and reveals her name to be Ava.
Simon returns to the SM-8 and uses Ava's credentials to download its data, including audio logs which confirm the light's connection to the Quiet Rapture, and that the blood ocean is composed of human blood. One of the SM-8's crew, the original source of the sea monster's voice, began obsessively drinking it before their sub was destroyed. Ava's vessel is caught by the monster; she apologizes for being unable to rescue Simon and begs him to protect the SM-8 data because it may save what is left of humanity. Simultaneously, the monster speaks to Simon again, telling him to destroy the data instead. The monster destroys Ava's vessel and attempts to kill Simon. Simon defies the monster and ties the Iron Lung's black box to a life vest, as the damaged submarine floods with blood and fleshy matter that rapidly grows and mutates his body. As the blood makes contact with the cracked medallion Simon holds, he merges with the seed within as it mutates into a giant tree, killing the monster as the submarine implodes inside its mouth. The life vest and the black box are shown floating on the surface of the ocean, discovered by an unseen party.
