The narrator of The Plague is obsessed with reaching objective truth. So much so, in fact, that he describes himself in the third person and won’t even tell us which character he is until the narration is complete. He uses "facts" and "data" that he’s obtained from various persons to keep things in perspective, and is convinced by his conclusion that he’s done just that. Camus’s intent here is that of irony; along with many existentialist thinkers, Camus believed there was no objective truth, or at the least, that we could never be certain of anything as objectively true. This, of course, means the narrator’s endeavor is doomed from the start, as is any attempt at "truth" in the world of The Plague.
That the narrator of The Plague cannot establish objective truth is a fact made irrelevant by the story he tells.
That the narrator of The Plague cannot establish objective truth renders this entire novel irrelevant.