Believable is a word used to describe something that can be easily accepted as truth, even by the skeptical mind. Given that the agents are a suspicious lot, this doesn't encompass a whole lot. Goodfic tends to be believable given the limitations of its source canon. Badfic, on the other hand, is often not. It is a notable quirk that events in the Real World are frequently not believable, but these are given more leeway than events in badfic. For Example: Seeing a pattern here? Let's hope so.