Mechanisms for Advancing the Plot
In presentational media, the plot of a story advances at the rate at which the reader reads or the display mechanism displays. In a video game, the storytelling engine causes the plot to advance but not at a fixed rate and not always triggered by the same mechanisms each time it advances. Different games use different triggers to tell the storytelling engine to move forward. In some games, succeeding or failing at a challenge triggers plot advancement. In others, the avatar's journey through the game world makes the story advance; in such games, entering a room or area may act as the trigger. In a very few games, the passage of time alone makes the plot advance, rather than anything the player does. The next three sections look at these mechanisms more closely. Each approach brings with it strengths and weaknesses, and you have to choose the one that works best for the story that you want to present.
In a good many games, the plot advances only when the player meets challenges or makes decisions. In a war game or a vehicle simulator, completing a mission or level might advance the plot. StarCraft advances the plot only when the player successfully completes a level, whereas Wing Commander advances its plot at the end of every mission, but the story proceeds in different directions depending on whether the player succeeded or failed. In both of these coarse-grained stories, neither time nor progress through space affect the plot, only the fact that the level has come to an end.
This system works well for games that involve no travel at all or those in which travel itself doesn't affect the plot. In a combat flight simulator, the player can fly all over the sky, but none of that travel influences the story. What affects the story is shooting down enemy planes or being shot down by them.
Suspended, a text-based puzzle game from Infocom, also uses this mechanism.
All game and story events take place in a restricted area, a small group of rooms. Solving puzzles in the different rooms causes the plot to advance.
Sometimes the trigger for advancing the story isn't surmounting a challenge but making a choice or decision. Role-playing games often give the player important decisions to make, such as whether or not to join a particular guild, the consequences of which significantly affect the story. Once the player makes a decision— and decisions are often irreversible—the plot advances.
If you require the player to succeed at challenges in order to advance the plot, the storytelling will be jerky, with sudden stops and starts. The player will sense that the story stalls every time he's stymied by a challenge, then starts up again when he meets the challenge. That doesn't matter much in coarse-grained stories—the player only expects storytelling at long intervals anyway—but in fine-grained ones it feels rather mechanical.
Adventure games and role-playing games use this approach, but they combine it with avatar travel as a means of triggering plot advances, somewhat reducing the mechanistic feel of the plot advancement. They treat the story as a journey, which is the next topic.