In the late 19th century, the cargo ship Demeter sets sail from Carpathia to London, entrusted with transporting fifty unmarked wooden crates. As the voyage begins, unusual occurrences plague the ship. Clemens, a doctor aboard for the journey, notices strange happenings linked to the crates in the ship's hold. Anna, a young stowaway, is discovered amidst growing unease among the crew. Her presence onboard deepens the mystery as she claims to possess knowledge about a malevolent entity hidden within one of the crates. Captain Eliot, a steadfast sailor nearing retirement, grapples with keeping his crew focused and safe despite mounting fears.

As days turn to nights on the treacherous sea, horrifying events escalate. Crew members vanish mysteriously, and tension rises as suspicions of sabotage or mutiny take root. The ship's livestock is found slaughtered in a grotesque fashion, implicating something sinister aboard. Clemens and Anna grow increasingly determined to uncover the truth before it's too late, realizing they must contend with an ancient evil that threatens not only their lives but the entire crew. With the vessel's destination drawing closer, they face a desperate race against time as night after night the terror creeps closer and survival becomes uncertain.