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.
- Severus Snape shagging Remus Lupin is not believable.
- Aragorn as a misogynistic bastard is not believable.
- Sesshomaru falling in love with a random female instantly is not believable.
- On the other hand, Glorfindel slaying a Balrog is perfectly believable, as it has happened in canon.
- Same goes for Harry Potter slaying a full-grown basilisk at age twelve, albeit with some help.
Seeing a pattern here? Let's hope so.