Source Player Core pg. 7
Characters and their choices create the story of Starfall, but how they interact with each other and the galaxy around them is governed by rules. So, while you might decide that your character steals a starship and takes off on a perilous interstellar journey, their abilities determine your character’s chance of success, the choices you make, and the roll of the dice. The GM determines the premise and background of most adventures, although character histories and personalities should also play a part. Once a game session begins, the players take turns describing what their characters attempt to do, while the GM determines the outcome, with everyone working together to create the story. The GM also describes the environment, other characters’ actions, and the events. For example, the GM might announce that the PCs’ home world is under attack by the BioCommune. The characters might fight off the first strike, then track the invaders back to a planetoid in the Frontier—just as a raiding party arrives! The PCs can choose to battle emganats, fend off invaders, or do both. Whatever they decide, their success ultimately depends on their choices and the outcome of their die rolls.
A single narrative encompassing the setup, plot, and conclusion is called an adventure. A series of adventures creates a larger narrative, known as a campaign. An adventure might take several sessions to complete, whereas a campaign might take months or even years!