Now that the speaker has told us about how his heart has gotten suckered by his eyes, he switches focus from his eyes to his heart.
Before we get into what the speaker actually says, we should clear up the meaning of the term "several plot," which is bound to cause some confusion. Why?
Because neither of the words is being used according to its most common modern meaning.
First of all, "several" doesn’t mean "a few" or "a small number." Instead, here it means "separate," "set aside," or "private." (You can think of the "sever-al" part of the word as meaning that this thing is "severed" from everything else.)
Also, "plot" doesn't mean the "story" of a movie or book. Nope. It means "plot of land," like in a cemetery, or a garden.
So, what the speaker is really asking here is: "Why does my heart think that that is a private plot of land?" Note that we don’t know which plot of land he is referring to. We’ll have to wait until the next line to get that sorted out.
But let’s not rush ahead too quickly. First, let’s see if we can figure out what the speaker is talking about here. What in the world does he mean by a "several plot"—or, as we’ve just rephrased it, a "private plot of land"?
If you guessed that this was going to require some work from your inappropriate imagination, you guessed right.
The "plot" he is referring to—just like the "bay" back in line 6—is a metaphor for the most intimate part of his lady friend’s anatomy.
Thus, what the speaker is really asking is why his heart thought that the most intimate part of his lady friend’s anatomy—the "plot"—was "several" (i.e., "private" or "set apart"), which we can safely assume means "set apart for him."
Which my heart knows the wide world’s common place?
Yikes. The speaker throws out another zinger. Before, he told us that his eyes were "anchor’d in the bay where all men ride," now he asks why he considers something to be a "several plot" (10) which is actually the "wide world’s common place."
Clearly, the speaker is exaggerating a bit here—there’s no way that his lady friend could actually have slept with the entire world, is there? But it still is an effective way of getting his point across: the lady gets around.
The hyperbole (exaggeration for rhetorical effect) of this line might be a sign of how emotionally upset the speaker is.
Or mine eyes seeing this, say this is not,
This line follows from the same "Why" that asked at the beginning of line 9.
The speaker has just asked why his heart stubbornly keeps believing that his lady-friend is sexually monogamous with him (9), even when his heart knows that she is sexually promiscuous with the "wide world" (10). Now he wants to know why his eyes, which are aware of what’s going on (they are "seeing this"), keep pretending that this isn’t the case ("say this is not.")
So, eyes can speak? In a poem (and with a little help from personification) they can.
To put fair truth upon so foul a face?
The general idea of this line is that the speaker continues talking about how his eyes have deceived him.
We can tell that the speaker's not pleased, just by reading this line out loud. Hear all those F sounds? That, friends, is alliteration. That's the term for stringing together words with the same starting consonants (in this case: F).
The F sound gives off an explosion of breath, and here we get to really here the speaker's frustration (another good F-word) take shape in this word choice.
As far as the actual phrasing of the line, that's a bit tricky, mainly because he uses the words in some unexpected and sort of old-fashioned ways. Here, it seems that Shakespeare uses truth to primarily mean "loyalty."
So, what does it mean to "put fair truth" (a.k.a. loyalty) "upon" something?
We think that what Shakespeare is getting at here in the words "put […] upon" is the idea of "putting a mask" or "a new coat of paint" on something.
Basically, he’s talking about making something look loyal that really just isn’t.
But what is being made to look loyal? The poet tells us that "fair truth" is being put "upon so foul a face."
We already know, from earlier lines in the poem, why the "face" of the lady is so foul: because her alleged sexual promiscuity has made her seem disgusting in the eyes of the speaker. Here, the speaker is using face not in a literal sense, but as another metaphor for the beloved's infidelity.
As well, the speaker is saying that his eyes are making something—his mistress—seem loyal, when, in fact, she isn’t loyal.
But wait a second. Doesn’t this mean that the "truth" that the speaker’s eyes "put […] upon" his mistress’s "face" is actually… false?
It sure does. And you can be absolutely sure that Shakespeare intended for us to be scratching our heads in this way. You’d be hard pressed to find any other writer who takes such delight in wordplay, especially of the mind-bending variety.
Of course, it’s just like Shakespeare to be punning on two meanings of the word "true" (true meaning real, and true meaning faithful). The hidden joke of the line—that it’s "false" for the speaker’s eyes to see "truth" (fidelity) in his mistress—also plays on the more common meaning of "truth" (real or actual).