The speaker uses the occasion of a flea hopping from himself to a young lady as an excuse to argue that the two of them should make love. Since in the flea their blood is mixed together, he says that they have already been made as one in the body of the flea. Besides, the flea pricked her and got what it wanted without having to woo her. The flea’s bite and mingling of their bloods is not considered a sin, so why should their love-making?
In the second stanza the speaker attempts to prevent the woman from killing the flea. He argues that since the flea contains the “life” of both herself and the speaker, she would be guilty both of suicide and a triple homicide in killing it.
The woman in question is obviously not convinced, for in the third stanza she has killed the flea with a fingernail. The speaker then turns this around to point out that, although the flea which contained portions of their lives is dead, neither of them is the weaker for it. If this commingling of bodily fluids can leave no lasting effect, then why does she hesitate to join with him in sexual intimacy? After all, her honor will be equally undiminished.
Donne here makes use of the wit for which he eventually became famous—although in his own day his poetry was often considered too lurid to gain popular notoriety, and little of it was published during his lifetime. One of his earlier poems, “The Flea,” demonstrates his ability to take a controlling metaphor and adapt it to unusual circumstances. “The Flea” is made up of three nine-line stanzas following an aabbccddd rhyme scheme.
He begins the poem by asking the young woman to “Mark this flea” (line 1) which has bitten and sucked blood from both himself and her. He points out that she has “denied” him something which the flea has not refrained from enjoying: the intimate union of their bodily fluids (in this case, blood). This commonplace occurrence, he argues, “cannot be said/A sin, nor shame, nor loss of maidenhead” (lines 5-6); if this tiny commingling of the two people is not wrong, then how can a greater commingling be considered evil or undesirable? He even points out that the flea is able to enjoy the woman’s essence “before he woo” (line 7), the implication being that he need not court the woman in order to enjoy her sexual favors.
In the second stanza the poet argues for the life of the flea, as his desired lady has made a move to kill it. He paints the flea as a holy thing: “This flea is you and I, and this/Our marriage bed, and marriage temple is” (lines 12-13). (Note also the reference to the Christian concept of "three lives in one" (line 10), suggesting that a spiritual union already exists, although unlike a spiritual marriage in a "marriage temple," the third being in the trio is not God but a flea.) Besides arguing for the sanctity of the flea’s life, the speaker is also arguing that he and the lady have already bypassed the usual vows of fidelity and ceremony of marriage; thus, he pushes toward his point that the two of them have already been joined as one in the flea, so there is no harm in joining their bodies in sexual love.
There is a hint that he has already attempted to gain the lady’s favors and failed, either through her response or that of her parents: “Though parents grudge, and you,” (line 14) he says, suggesting that even her opinion does not matter anymore. The flea has already “cloister’d” them within its body’s “walls of jet” (line 15, possibly also suggesting that they are alone together in a dark room). The woman’s disdain for him and his suit becomes more apparent as he claims she is “apt” to kill him (line 16), following her habit of killing fleas, but he offers that she should refrain from harming the flea because in so doing she would add suicide (“Let not to that self-murder added be” line 17) by destroying the vessel holding her blood. In fact, he says, she would be guilty of “sacrilege, three sins in killing three” (line 18) since his own blood is there too.
He fails in his defense of the flea, for she has “purpled” her finger with the flea's blood by the opening of the third stanza (line 20). It is a “sudden” but perhaps inevitable betrayal of an innocent being. The woman claims triumph over the lover's argument, responding that neither she nor the man is weaker for her having killed the flea (lines 23-24). In this way she attempts to unravel the speaker’s argument that the flea represents a sacred bond between them; the flea is simple to kill and nothing has been lost, and the single drop of blood will not be missed. Thus there is no reason to have sex.
The poet, however, is quick-witted enough to turn her argument back against her: if the death of the flea, which had partaken of just a tiny amount of their life-essences, is virtually no problem, despite his pretended fear, then any fear she might have about her loss of honor is equally a “false” fear. The act of physical union would cause virtually no serious harm to her reputation. That is, as much as she lost to the flea, “Just so much honour, when thou yield'st to me, / Will waste, as this flea's death took life from thee” (lines 26-27). He thus returns to his original argument from the first stanza: the flea’s intimate contact with the woman has caused her no harm, so a physical encounter with the poet will cause no harm either.
Although the lover suggests that he is in control and that it is a matter of "when thou yield'st," some feminist scholars have noted that he is powerless to do anything until the woman makes her decision. He merely utters his words of warning, but she can raise her hand and kill the flea; similarly, she can exercise her power by continuing to deny the man his desires. The flea could take what it wanted without stopping to woo, but the lover uses no force beyond the force of argument. He has not been successful so far, but we do not know what will happen next.