Quake is technically divided into seven episodes. The starting episode, Welcome to Quake, is a hub that connects to all other Single Player episodes. The final level of the Single Player game is also part of another episode, Final Level. While both are treated as episodes with only one level, they do not have numerical values like the other Single Player episodes, meaning that the Single Player game is generally treated as having four episodes.
If following numerical order, the Single Player game is made up of four episodes of 7 to 8 separate maps (including one secret level per episode). The player must complete each episode by playing each of the levels from start to finish before moving onto the next episode. The four episodes can be completed in any order but it is best to play them in order as they get progressively more difficult. Once all Episodes are completed and the player holds the four runes, the final level of the game will open, allowing the player to take on the witch-goddess, Shub-Niggurath.
The four episodes of the game take place in different dimensions that apparently have been overrun by Shub-Niggurath's children. The goal of each episode is to find one of the magical Runes, the four of which can be combined in order to open the gateway to the witch-goddess to fight her in her otherworld cathedral. In the first level of every episode, the player attempts to reach the Slipgate that will lead them to the desired dimension by fighting through the military base which has become overrun with humanoid Grunts, the stronger Enforcers, and Rottweilers. Upon retrieval of the Rune the player must take the nearest Teleporter leading back to their world before hunting down another Slipgate.
Another episode exists that is not part of the Single Player game, Deathmatch Arena. This, as can be inferred from the title, is made up of Multiplayer levels and is therefore not considered part of the Single Player campaign. This episode is mainly designed as an easy way to categorize all of the Multiplayer levels and place them in a location allowing for easy access.