From the Cherry Picked Games site:
In the post-demon world, humanity protects itself in the shells of metropolitan cities connected by rail. The town of Flagstaff hosts brave mercenaries dedicated to protecting these trains and keeping the demons at bay. "Fourth World" follows the players as they arrive in Northern Arizona and become embroiled in local politics, sinister plots, and ancient secrets that may save us once and for all.
This guide aids game masters in directing a Catalyst campaign. "Fourth World" takes 3-5 players through a longer, 7-session campaign.