From Below

A seven-person crew aboard the research vessel Skipjack prepares to dive to the wreck of the SS Arcadia, an ocean liner that vanished during a transatlantic voyage in 1928 with 265 crew and 409 passengers. No bodies were ever recovered, and a single distress call that provided contradictory coordinates left the ship's fate one of maritime history's most enduring mysteries. The wreck has been located 312 feet below the surface of the Gulf of Bothnia, between Sweden and Finland, hundreds of miles from the Arcadia's intended course.

Cove Waimarie, the expedition leader and documentary host, assembles her team for a dive sponsored by Vivitech Productions, which requires a minimum number of hours of footage from inside the ship. Before the first dive, the remotely operated vehicles (ROVs), main cameras, and navigation system all malfunction. The crew includes Roy, the camera technician; Aidan, a nervous 22-year-old assistant who barely passed his dive certification; Hestie Modise, a marine biologist; Sean, the ROV operator; Vanna Ford, a taciturn dive safety officer with over 4,000 logged dives; and Devereaux, the ship's owner and a historian. Cove privately acknowledges that her team is dangerously underprepared and that she needs this job: She filed for bankruptcy after paying for an experimental treatment for her dying father that failed.

The narrative alternates between the modern dive team and historical chapters set aboard the Arcadia during its final voyage in April 1928. Through multiple perspectives, including Harland, a young sailor, and Fourth Officer Swain, these sections trace the ship's descent into chaos. A foul-smelling fog envelops the vessel. Crew members hear scratching and tapping inside the walls. Fitz, the stock master, tears open the interior hull and reveals a mummified corpse wedged between the double layers of plating, likely a shipyard worker who died during construction. Passengers begin walking onto the fog-shrouded deck in trances and throwing themselves overboard. A phenomenon called "burrowing disease" takes hold: Dying crew members wedge themselves into wall cavities and gaps beside furnaces. The captain orders windows boarded shut and passengers locked in their rooms, but the things inside the walls begin to emerge.

On the first dive, the team enters the Arcadia through a gash in the hull. Inside, the oxygen-free, near-freezing water has preserved the ship with extraordinary fidelity. Cryptic messages cover the walls, including "They came through here" with an arrow pointing downward, and "Do not let him out." A hallway has been blockaded with furniture and rope. When Roy tears open the curtains of a dining alcove, a preserved human body drifts out toward the divers. The team panics and flees. Back aboard the Skipjack, Hestie explains that the body is real, its skin coated in adipocere, a waxy substance formed from body fats in the absence of bacteria.

Meanwhile, Sean discovers Vanna's private journal, which contains disturbing poetry about watching people drown. Cove learns that Vanna's dive partner and life partner, Abby Freeman, died in a cave-diving accident 18 days earlier, and the entries are a therapeutic exercise to process grief. Cove keeps Vanna on the team as the only experienced deep-water diver.

The second dive grows more dangerous. Cove's team becomes trapped in a cargo hold after a silt-out, a blinding cloud of disturbed sediment, fills the water. They escape into an adjacent hold containing at least 14 shrouded bodies stored during the voyage. On the upper levels, Hestie and Vanna find that their dive line, the guideline rope divers follow back to safety, has come untethered. In the ship's hospital, Hestie finds rows of patients still strapped to their beds and has a seizure. Vanna holds her steady until the others arrive and carry Hestie out.

On the third dive, Cove and Roy explore the upper decks, finding a journal cataloguing deaths from "burrowing disease." Aidan, paired with Vanna, secretly deviates from the plan at Devereaux's request to retrieve a fabric sample from the cargo hold. As he cuts the cloth, a shrouded corpse opens a milky eye and seizes his wrist. Vanna severs the creature's hand with her dive knife.

The Arcadia's dead begin to rise. Cove and Roy encounter corpses blocking a hallway, joints clicking as they advance. One seizes Cove's mask and begins strangling her before Roy drives it back with a flashlight. They discover the dead recoil from bright light. The creatures herd the divers deeper into the ship, blocking every exit except downward. In the swimming pool, hundreds of intertwined corpses stir and rise. Trapped in a changing room, Cove kills a creature and squeezes through a hole in the wall to reach the open ocean. Vanna and Aidan's escape is equally desperate: In the boiler room, Aidan sinks toward buried corpses that seize him and tear his suit. Vanna inflates both their buoyancy compensators, inflatable vests that control a diver's depth, to maximum, tearing him free.

Back aboard the Skipjack, Devereaux reveals his theory: The cargo hold contained fabric from Prescott Textiles, whose manufacturing process produced a neurotoxin that caused paranoia and hallucinations. He believes the off-gassing poisoned the crew over the course of the voyage. Hestie adds that the toxin alone does not explain what they witnessed; the ship was already cursed, and the crew's escalating fear activated whatever malevolence lay dormant inside it.

That evening, Aidan goes missing. He has taken a spare dive suit back to the Arcadia to recover his engagement ring, a family heirloom from his girlfriend Penny's parents that he lost inside the ship. Cove, Vanna, and Roy enter the water for a rescue. Roy confesses over the radio that he disabled Sean's ROVs so the team would dive rather than rely on remote exploration.

In the boiler room, they find Aidan inside a towering pillar of intertwined corpses. Vanna charges into the mass to connect a spare air canister to his failing supply. Cove and Roy drop underwater flares, and the bright light forces the bodies apart. In the dining hall, the dead surround them and block the skylights. Sean, having repaired an ROV with the chips Roy stole, pilots it into the hall. Its floodlights drive the dead back, and the team escapes. Vanna is the last out, clinging to the ROV before its battery dies.

Five months later, Cove appears on a talk show promoting the documentary, which has been edited to exclude all footage of the reanimated dead. The crew has agreed to suppress the evidence and is lobbying to have the wreck declared an off-limits grave site. Aidan sends a group photo: Penny said yes to his proposal. Cove reflects on how the Arcadia dismantled her understanding of death but pushed her toward growth, and she resolves to face the future one step at a time.