The year is 1913. The steamship SS Atlantica is two days out from port on its voyage across the Atlantic Ocean. Its unsuspecting passengers fully anticipated a calm journey to Boston, Massachusetts, with nothing out of the ordinary to look forward to. However, strange nightmares plague the minds of the people aboard the ship every night; rumors circulate of dark shapes following closely behind the ship just beneath the waves; and tensions rise when a body is discovered in the ship's chapel, signs of a strange ritual littered around the corpse.

Lurking within the depths of the Atlantic Ocean are a swarm of vicious, unspeakable horrors: the Deep Ones, led by Mother Hydra and Father Dagon. For reasons unknown, they have set their sights on the Atlantica, and their minions, taking the form of human-Deep One hybrids, have infiltrated the steamship to help sink it from within. Each game of Unfathomable has one or more players assuming the role of one of these hybrids, and how well they can secretly sabotage the efforts of the other players might mean the difference between a successful voyage and a sunken ship.