The game takes place in the Zork calendar year 948 GUE (although the passage of time is not notable in gameplay). The player steps into the deliberately vague role of an "adventurer". The game begins near a white house in a small, self-contained area. Although the player is given little instruction, the house provides an obvious point of interest.

When the player enters the house, it yields a number of intriguing objects, including a brass, battery-powered lantern, an empty trophy case, and an Elvish sword of great antiquity. Beneath the rug a trap door leads down into a dark cellar, which is revealed to be one of several entrances to a vast subterranean land known as the Great Underground Empire. The player soon encounters dangerous creatures, including deadly grues who only prey on their victims in the dark, an axe-wielding troll, a giant cyclops who cowers at the mention of Odysseus, and a nimble-fingered thief who makes mapping the maze difficult by removing or scattering any items that the player might drop to leave a trail.

The ultimate goal of Zork I is to collect the Twenty Treasures of Zork and install them in the trophy case. Finding the treasures requires solving a variety of puzzles such as the navigation of two complex mazes and some intricate manipulations at Flood Control Dam #3.