When something is believable, people believe it. It's believable when the story is plausible and when the characters are believable.
The story was not believable because the characters were not believable.
The story was not believable because the plot was not plausible.
noun
the quality of being believable or trustworthy
Synonyms:
1
People who believe in optimism think positively.
2
I don't think that people who don't believe the argument should garble it.
3
People are strongly believed in verity.
4
People believe that the rumor is picayune.
5
Some people believe in Christ.
6
The people believe in protecting the environment.
7
He was a great believer in a true people to people rapprochement.
8
He was a great believer in a true people to people rapprochement.
9
Many people tend to believe in atheism.
10
People believe that ghosts are wandering in the house.