The phrase "a plain truth" is correct and usable in written English.
It can be used when stating a fact that is clear and straightforward, often implying that it is obvious or universally accepted.
Example: "It is a plain truth that hard work leads to success."
Alternatives: "an obvious fact" or "a simple truth."
"There was a right side and a wrong side," Douglass insisted, a plain truth that could not be smothered by placing flowers "alike and lovingly, on rebel and on loyal graves".
Well, let me tell you a plain truth: there isn't.'.
At the same time, it is a plain truth of the Holmes stories that Holmes might never have become a detective, that this was a purely contingent choice on Holmes's part.
This is no criticism, just a plain truth.
He characterised Seacole as "a plain truth-speaking woman who has lived an adventurous life amid scenes which have never yet found a historian".
In 1933 he established an independent radio ministry in Eugene, Ore., the Radio Church of God, and a magazine, The Plain Truth, to disseminate his ideas.
Despite all the rhetoric, much of it self-created, of Franklin as natural man, pragmatic man, empirical man, man with a kite, the plain truth is that his genius was, in every realm, for airy abstraction brought to earth, the single-fluid theory of life.
There's a good reason for this: Whether you're a social butterfly or a bookish introvert, the plain truth is that approximately 80% of jobs are acquired through networking.
In a perfect inversion of plain truth, the Royal Bank of Scotland recently assured from billboards that it is "Here For You".
This comes in the form of an event or a situation when we have to choose between the plain truth or... a "twisted" one.
You feel, if it weren't for the intervention of his uncle, he would wreak terrible revenge on the common soldier who tells him a few plain truths the night before Agincourt.
