From the introduction:
Before discussing the playbooks, consider this:
This game is about Fighting Some Evil that threatens the characters, or who they love, or the place they live in, or the Eternal City or the Seven Nations and so on. This game is the story of some dark, obscure threat, and of the heroes that have set on a journey, on a mission to defeat it.
This will be the focus of the first adventures, which can last only a few sessions, or an entire campaign if you feel like it. Once the dark threat will be defeated, or somehow neutralized - for good or maybe just for a while - you can spin the game in other directions.
But start with this, with a dark threat.