Like many works by the Metaphysical Poets, "The Flea" contains wild shifts in the imaginative setting of the poem (the images you think about as a reader), even as the literal setting stays in one place. Literally, the poem is set anywhere you might find fleas, which in Renaissance England included...everywhere. Seriously, we're sure there were fleas even in the royal palace. No exterminators back in the day, you know.
Imaginatively, the poem begins by zooming in on the woman's smooth, pale arm. Hear the buzzing of the flea as it lands on her skin and begins to delicately suck her blood. The insect swells up like Violet Beauregarde turning into a giant blueberry in Willie Wonka and the Chocolate Factory. Next we actually go inside the flea, where a marriage is consecrated with the mingling of two bloods. See the bride's parents fuming angrily in the aisles of the church...hey, wait a minute, are those black church walls...moving? Yes, the church is aliiiiive, and those walls are nothing but the insides of the flea.
Again the poem moves outside the flea. The woman's hand comes down – smack – on the bloodsucking flea, coloring one of her nails "purple" with its blood and hers. The speaker looks aghast, as if the world has just ended, but he gets over it a moment later.