From the introduction:
The game master (GM) is the member of your gaming group who narrates events and arbitrates the rules. Though simple in overview, the role can be both challenging and rewarding in application.
With each of the other players directing a single character, it is up to the GM to set the stage, provide the motivation, description, and voice of every other character in the world. They must lay a path for the players to follow and adapt it seamlessly when the heroes choose a lateral approach. They must litter the heroes journey with worthy challenges, desirable treasures, and evocative revelations. They must provide a villain too terrible to face and a cause too vital to abandon.
And that’s just to narrate.