The story is set in 1913 aboard the steamship SS Atlantica, which is traveling toward Boston, but the peaceful voyage soon turns into a nightmare. Passengers are tormented by terrible dreams, dark shapes appear around the ship, and a body bearing signs of an occult ritual is discovered on deck. The ship is besieged by the Deep Ones, led by Mother Hydra and Father Dagon, and their hybrid infiltrators can be found among the crew and passengers themselves.

The main tension comes from hidden roles — some players are humans trying to save the ship, while others are hybrids or cultists secretly working to sabotage the journey from within. This creates a paranoid, eerie atmosphere strongly reminiscent of Lovecraftian eldritch horror.

Each player receives a loyalty card revealing whether they are human or a saboteur. These hidden roles can change mid‑game, so you never truly know whom you can trust. Players share four key resources — fuel, food, sanity, and souls. Humans aim to preserve these resources, while saboteurs attempt to deplete them secretly. At the end of each turn, a player triggers a crisis that usually requires a group decision or a skill test. In skill tests, players may secretly add cards, allowing saboteurs to sabotage the outcome without revealing themselves.

The ship is attacked by Deep Ones, who appear in different parts of the deck, forcing players to move through rooms, fight creatures, rescue passengers, or repair damage. The humans’ goal is to survive long enough for SS Atlantica to complete its voyage, while the hybrids aim to sink the ship or exhaust its resources before it reaches its destination.

Unfathomable is a tense, thematic, and highly interactive game that thrills players who enjoy hidden roles, eerie atmosphere, and the constant feeling of paranoia. 🐙