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

Lurking beneath the waves 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 steamship 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 taking on 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.

If you're a human, you need to fend off Deep Ones, keep the Atlantica from taking too much damage, and carefully manage the ship's four crucial resources if you want any hope of making it to Boston, all while trying to figure out which of your fellow players are friends and which are foes. Everyone shares the same resource pool, but humans try to preserve them while traitors will strive to subtly deplete them. Being able to tell when someone is purposefully draining the group's resources is harder than you think, especially when you take crises into account!

At the end of each player's turn, that player must take a mythos card. Each of these cards represents a crisis that the whole group must attempt to resolve together. Some of these crises, such as "Food Rationing", call for a decision that could potentially put the ship's passengers or resources at risk, while others, such as "Hull Leak", call for a skill test in which failure could have disastrous consequences.

During a skill test, every player contributes skill cards from their hand to a face-down pile shared by the group. Once everyone has contributed (or decided not to), the cards are shuffled, then revealed. If enough of the correct skills were contributed, then the group will pass the test! But if the wrong skills were contributed, they can actually hinder the results and lead to failure. Thus, skill tests can be dangerous opportunities for traitors to sabotage the humans' efforts, so you have to stay on your toes at all times.