

To acquire the items, the player usually is not able to acquire them at that island and must go to different islands or defeat sea beasts.

To progress in the quests, the player has to provide an item or complete a skill check. The Unterzee has multiple islands, with each island having its own storylet quests. This, accompanied by the music creates an atmosphere of tension. The actual gameplay consists of two modes- the player controlling the movement of the ship on the map with combat taking place in the same mode, and a story mode where the player is presented with several choices (sometimes accompanied by a skill challenge) in a card-like format. There are several roguelike gameplay elements, such as partially randomised maps and permanent character death, but subsequent characters can inherit some of their predecessor's possessions, and a player may create a will to ensure lodging and wealth for their successors. Resources to achieve these ends are acquired by discovering new locations, trading goods across the Unterzee, battling ships and "zee monsters", and completing "storylet" quests. The player can win by achieving their chosen ambition, such as becoming Fallen London's most celebrated explorer or amassing enough wealth to retire. The player takes on the role of an Unterzee steamship captain, whose background and ambitions are customisable.
