If you allow the player to influence future events and change the direction of the story, then the story is nonlinear. This chapter examines two of the most common structures for nonlinear stories—branching stories and foldback stories—in detail in the next two sections. A third approach, emergent narrative, is more of a research problem than a standard industry mechanism, and we'll discuss it briefly. After that we'll look at a new hybrid technique that shows great promise for the future of interactive storytelling. Finally, we'll study an important issue for any teller of nonlinear stories: How many endings should the story have?
A branching story allows the player to have a different experience each time he plays the game. The story offers not one plot line but many that split off from each other at different points. As the designer, you decide on the different possible plot lines and how they relate to each other. During play, the storytelling engine keeps track of which plot line the player is following at any given time. When the story reaches a branch point—a place where the current plot line subdivides—the core mechanics must send a trigger to the storytelling engine to tell it which of the possible branches of the story the player will follow next.
Game events—either player events or in-game events generated by the core mechanics (such as an action taken by an AI-driven NPC)—determine which branch the story will take. Player events that influence the direction of the story fall into two categories: efforts to overcome a challenge or decisions that the story asks the player to make. Branch points connected with player decisions have one branch for each option that you offer to the player. Typically, branch points associated with challenges have only two branches leading on from the branch point, one for success and one for failure, though you can also create different numbers of branches for different degrees of success if you want to. We'll consider the emotional consequences of branches based on challenges versus those based on choices in the later section "Endings."
If an event in the game causes the plot to branch right away, that event has an immediate influence on the story. This is the most common kind of branch and the easiest to implement. The player makes an irrevocable decision—which road to take, for example—and the story promptly reflects his choice.
However, sometimes the player can make a decision early in the game that influences a branch point much later, in which case that decision has deferred influence, or he can make a whole series of decisions throughout the game that cumulatively affect a branch point, such that his actions and decisions, taken together, have cumulative influence.
If you use deferred or cumulative influence, you must make it clear to the player what the possible consequences of his decisions will be. It's unfair to give the player a choice early in the game without warning that this choice will have long-term repercussions, and then change the direction of the story hours or days later based on that choice. Furthermore, if he wants to change his mind, he has to reload the game all the way back at the point where he first made the choice, choose differently, and then play all the way through the game again. (That's assuming he still has a save point there to reload from.) And he can only do this at all if he realizes how his decisions affected the current branch, which may not be obvious.
For example, if you allow a player to choose right at the beginning of a role-playing game whether he will play as a healer character or a fighter character, you should tell him that such an important choice will have significant deferred consequences throughout the game.
Many role-playing games use cumulative influence to build up a sort of reputation for the player. The game keeps track of the player's behavior over time, and if the player consistently performs evil deeds, the NPCs in the game begin to treat him as an evil character. Again, you should warn the player that his cumulative behavior will have consequences later in the game.
Trivial decisions—which color hat will I wear?—should have only trivial consequences. If a trivial decision has a profound consequence, the player will feel cheated: He didn't know that the decision mattered and had no reason to expect it to matter. Attaching important consequences to trivial decisions violates the requirement that stories be credible and dramatically meaningful. The Hitchhiker's Guide to the Galaxy, a text adventure, did this for comedic and ironic purposes, but most players and critics judged it to be an unreasonably difficult game for exactly this reason: The player couldn't predict what the consequences of his actions would be.
Give players a reasonable amount of information about the possible consequences of their decisions, especially if the decision's consequences are deferred, so that they can make informed choices. Don't tie important consequences to what seem to be trivial decisions.
A diagram of a branching story looks somewhat like a tree, although by convention the root—the beginning of the story—appears at the top, so that the tree branches out as it goes down the page and the story goes forward in time. Figure 7.2 shows a small part of the structure of a branching story.
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Each of the circles in Figure 7.2 represents a branch point, and each arrow represents a branch, that is, the player's movement along a plot line to the next branch point. The storytelling engine keeps track of the player's position in the story at any given moment.
As you look at Figure 7.2, be sure to note the following:
■ The branch points don't always have the same number of branches leading away from them. A story can branch in any number of directions at any given point.
■ The branches go down or sideways, but they never go back up again. The diagram depicts the possible progress of a story, and stories always move forward in time, never backward. In the course of playing a single game, the plot never follows the same branch or passes through the same branch point twice. This enforces the rule that stories must not contain identical repeating events and helps avoid the risk of continuity errors, as discussed earlier.
■ Unlike branches on a real tree, different branches can merge; that is, different plot lines can converge. Many branch points can be reached by more than one path.
■ The diagram depicts two possible endings that may be reached by different paths. The complete diagram would show additional endings farther down.
■ The diagram shows only one start point, but in fact a story could have several start points if the player made a key decision before the story actually began. The player might select one of several different characters to be his avatar, and that choice could determine where the story begins. Or the storytelling engine could choose from among several designated start points at random just to make the beginning different each time the player plays the game.
The branching story mechanism is the classic method for creating interactive stories that give players lots of agency. Branching plot lines let you tell a story in which the player's actions strongly affect the plot, and he can see the effect of his actions if he plays the game more than once and makes different decisions the second time through.
Be aware of the following three serious disadvantages of the branching story mechanism before you decide to use that structure for your game's story.
■ Branching stories are extremely expensive to implement because each branch and each branch point require their own content. In Figure 7.2, a player can experience at most six branch points in playing from the top to the bottom of the
figure—not very many. That represents six player choices or challenges. After six choices—for example, to take the left fork of the road, to enter the building, to go upstairs instead of down, to talk to the old woman, to accept a letter she offers, to leave the room—the player has barely started the game. Yet even this simplified
example involves 21 branch points and 35 different branches, each of which requires its own story content: gameplay and narrative material. If none of the branches merged again, there would be even more. This rapid growth in the number of branches is called the combinatorial explosion. (Combinatorics is the field of mathematics that studies the number of possible combinations of a set of things—in this case, a set of branch points in a branching story.)
As a result, most modern games don't actually include much branching, and they often include long periods during which the player plays but doesn't change the story. Wing Commander, a space combat simulator, contained a branching story, but it branched only between missions, not during them. Eventually, the Wing Commander series abandoned branching storylines entirely because they proved to be too expensive.
■ Every critical event (those that affect the entire remainder of the plot) has to branch into its own unique section of the tree. Suppose a character can live or die at a particular branch point. If he dies, he must never be seen again, which means none of the plot lines from his death onward can include him. His death requires an entirely separate part of the tree that can never merge back into the rest—otherwise, he might reappear after the player knows that he's dead. If this happens with two characters, the game requires four separate versions of the story: a version in which both live; a version in which both die; a version in which A lives and B dies; and a version in which B lives and A dies. Again, the number of possible combinations explodes.
■ The player must play the game repeatedly if he wants to see all the content.
If the storyline branches based on how well the player meets the game's challenges and he's very successful, then the next time he plays he has to play badly on purpose in order to learn the dramatic consequences of his failure! A lot of players would consider this to be absurd. They paid a great deal of money for the content in the game, and the only way to see it all is to play badly part of the time. This factor further contributed to the industry's abandoning stories that branch frequently.
If you want to make a branching story, you will have to plan out the structure in the concept stage of design. You should not actually write the story at that point in the design process, but you won't be able to plan a budget or schedule for your game unless you know how much content it will require, and a branching story's resource requirements expand very rapidly.
If you find that these drawbacks discourage you from using a branching structure, you can choose the compromise that the game industry most often uses when it creates nonlinear stories today: the foldback story.