The Flea is a lyrical poem in which a man tries to persuade a woman to become intimate with him. The poet, who is the narrator as well, uses a flea that has bit, both, the poet and his love interest as a metaphor of unity; of how the combined blood of his own and the girl's signifies a union no different than that which occurs during intimacy.
MARK but this flea, and mark in this,
How little that which thou deniest me is ;
It suck'd me first, and now sucks thee,
And in this flea our two bloods mingled be.
The tone shifts from romantic to argumentative because it is clear that the girl is not falling for it. Although theirs is a love grudged upon by their parents, and she may love him, she is not willing to sacrifice her honor. In fact, she wants to kill the flea at once.
O stay, three lives in one flea spare,
Where we almost, yea, more than married are.
This flea is you and I, and this
Our marriage bed, and marriage temple is.
Now it really becomes argumentative; apparently the poet is shocked at the fact that the girl ends up killing the flea and her hands are basically stained with the blood that, according to the poet, represents the union of their lives. In his dramatic eyes, the girl committed a murder. Notice, however, how the lack of dialog makes the reader infer that the girl is much less interested in intimacy than the poet; she i also not as dramatic, easily smitten, nor easy to convince. The verses that Donne "argues" with are precisely those mentioned above, which show the poet's main problem and the fact that he cannot find a solution, as the girl refuses to sleep with him.
The end of the poem is a cheap shot from the poet to the girl; she insists on keeping her honor, but according to the poet she had already lost it by "killing" all "three" of them i.e, the flea, and the union of their blood inside of it.
Just so much honour, when thou yield'st to me,
Will waste, as this flea's death took life from thee.
On display in Donne's idiosyncratic "The Flea" is the poet's talent for turning even the most improbable of images - in this case a blood-sucking insect - into an ornate metaphor of love and romance. In metaphysical poetry this is given the technical appellation of conceit, a kind of extreme analogizing in which two highly dissimilar objects are "yoked by violence together" (Samuel Johnson). Here and elsewhere, Donne uses the conceit as a vehicle for presenting an argument. While on the surface "The Flea" is the entreaty of a suitor to his resistant beloved, a closer examination of the poem reveals that through the structure of the conceit the poet is actually presenting points of a closely reasoned argument. Thus, his first point is this: sexual intercourse outside marriage is a sin. His second: but if the mingling of his with his beloved's blood can take place within the body of a flea which has bitten them both, then the act of intercourse loses its wickedness. Third: in what can only be described as an adroit shift in argumentation, the suitor makes the flea the entire world of the lovers. Yet, as the suitor nears the end of his case - where the symbolic becomes the reality upon which the suitor is quite prepared to act - the beloved clinches the argument by squashing the flea.