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: an ancient brass lantern, an empty trophy case, an intricately engraved sword, etc. Beneath the rug a trap door leads down into a dark dungeon. But what initially appears to be a dungeon is actually one of several entrances to a vast subterranean land—the Great Underground Empire. The player soon encounters dangerous creatures, including deadly grues, an axe-wielding troll, a giant cyclops and a nimble-fingered thief.

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 brutal mazes and some intricate manipulations at Flood Control Dam #3.