This is an argument against several Stoic positions are relates to the Sage and the conversion to wisdom. Plutarch takes issue, as most folks might, with the idea that all vices are equal, and if one has one vice you effectively have them all.
Who would deny the degrees of difference between a lie about a man's beard, or condemning Socrates to death? Or a lie to your boss about something in your personal life, and murder?
Our common sense experience of the world and the systems we've created in it recognize these distinctions. But the classic Stoics did not. Let's look at why that might be.
The problems with the common conception and Plutarch's argument are the external focus of them. The Stoic positions is not to be used in matters of jurisprudence, or punishment, or to correct the behavior of others. Rather, it's a tool for ourselves to correct vicious intent.
If we are trying to divest ourselves of vice, and instill virtue, then we must account for every wrong, no matter how small. The Stoic position that all evils (here as always, our own moral evil) are equal prevents us from deluding ourselves about the nature of our intent.
"Well, I may have lied to my spouse about this small thing, but at least I stopped doing something worse. So that's okay…"
The Stoic cannot with any intellectual integrity make such a justification.
Plutarch's opening assumption focuses on comparing the actions of two humans, which is an inappropriate use of the doctrine.
Despite that, and the general polemic nature of the piece, this discussion does tell us quite a bit about the Stoic positions which we don't see in many other places.
It's well worth the read time.