Many travel narratives follow an arc similiar to the plot of a story

The theme of a story is the main idea or an underlying meaning of a literary work that may be stated directly or indirectly. The plot of a story is the main events of a literary work.

Credibility is the term that refers to something believable, some information that can be trusted. Having this in mind, credibility is established when the descriptions, speech, actions, and events surrounding a character B. make sense in relation to the character. 
It is something logical, that anyone could accept as a given.