In a distant future, a cataclysmic event known as the Quiet Rapture severs all contact with colonized planets—star systems vanish, and humanity is reduced to a fraction of its former size. The story centers on Simon [Mark Fischbach], who is imprisoned for his role in the destruction of Filament Station. His punishment is to test and pilot the SM-13 submarine, nicknamed the Iron Lung, on a perilous mission: descend into an ocean of blood on a moon called AT-5 and return with proof that will grant him freedom if he survives. The sub is welded shut, the porthole sealed, and Simon’s only window to the outside world is a crude camera, with limited radio communications sparking frail contact with the surface.
As the dive begins, the sub’s sensors register movement outside, and Simon captures images of a skeletal figure on the ocean floor. Officials pull him up to review the find, then send him back down to collect a sample from the skeleton. Frustration gnaws at Simon—he wants out—yet he complies, only to discover that the “camera” is an X-ray device that irradiates the crew as it records. The return to the depths uncovers more unsettling truth: the mission is not what it seems, and the crew he thought he might save is suddenly endangered by the very instrument meant to document the discovery.
Down in the blood-soaked unhollows of AT-5, Simon uncovers audio logs from another pilot, proving he was misled about being the first to reach such depths. He attempts to inform Ava [Caroline Kaplan] of his strange sighting—an image of a living creature—but she urges him to press on and secure a sample. As he advances, the submarine fills with blood and an unidentified liquid, seeping into joints and circuitry, and his sense of reality begins to fray.
The isolation intensifies when Simon is knocked unconscious during the sampling. He awakens inside the sub to find movement in the camera feed, and he works to repair the vessel while fighting vivid hallucinations that threaten to erase his grip on reality. A wrecked submarine labeled SM-8 looms in the sonar, and its black box data remains inaccessible. A mysterious female voice speaks through the radio about a godly light beneath the ocean—an encounter that may be only a hallucination. In a nightmarish vision, the sub is destroyed, and Simon bursts to the surface to confront a vast eye filling a stormy red sky.
He returns to consciousness with Ava’s voice explaining that he has been missing for days—far beyond what his oxygen could have sustained. She commands him to recover SM-8’s data, promising to rescue him if he cooperates. Meanwhile, the interior of the Iron Lung fills with blood and fleshy matter, and the strange liquid begins to alter Simon’s skin, signaling that his body is mutating as the mission wears him down.
Reaching SM-8, Simon retrieves its data and uncovers a terrifying revelation: the blood ocean and its creatures are composed of the congealed remnants of humanity, a collective hive mind bent on absorbing any surviving humans. The weight of the discovery crushes him as the sub continues to fill with more organic matter. Ava cannot rescue him, she apologizes, and begs him to safeguard the SM-8 data because it may be humanity’s last hope. At the same time, the enigmatic voice urges him to destroy the data so that no one learns of the light beneath the waves. Before long, a hivemind creature destroys Ava’s vessel, severing the direct line of rescue.
With the sub’s corridors clogged by growths and the hull threatened by pressure, Simon tapes the black box data to a life vest and sabotages the Iron Lung, ensuring a catastrophic failure when the pressure builds. The vessel implodes after a final bite from the sea-born creature, killing both it and its pilot. An observer later discovers the life vest adrift in the blood ocean, the black box still intact, a chilling testament to the mystery that still lingers beneath the waves.
