Imogen is lamenting her unhappy situation, and wishes she had been stolen like her brothers. Pisanio enters with Iachimo, introducing him as a gentleman from Rome who has brought letters from Posthumus, who is safe. In an aside, Iachimo praises Imogen's beauty and admits that if her mind is as admirable, he has lost his wager.
Posthumus, in his letter, asks Imogen to treat Iachimo well in return for his "kindnesses" to Posthumus (line 23). She welcomes him. Iachimo appears to be wonder-struck by Imogen's beauty, and muses aloud how anyone could fail to distinguish between fair and foul. He seems to imply, without actually saying it outright, that there is another woman whom, in spite of her obvious inferiority ("sluttery," line 44), Posthumus is driven by lust to prefer to Imogen.
Imogen is mystified but changes the subject, asking after Posthumus's health and state of mind. Iachimo takes his chance to blacken Posthumus's character further, saying he is so merry that he is called "the Briton reveller". Imogen says he is more usually inclined to sadness; Iachimo replies that he has never seen Posthumus sad. On the contrary, he says, he laughs at a Frenchman who is pining after his girlfriend at home, for wasting time in such "bondage." He suggests that Posthumus is abusing his God-given bounty (both his own talents and his wife), and says he pities them. Posthumus neglects his "sun" (Imogen) and takes solace in a dungeon by the light of a burned-out candle, a a reference, perhaps, to a sordid and secretive place or to the worn-out slut whom he alleges Posthumus meets there.
Imogen asks Iachimo to speak more openly, as she would rather know the truth. Iachimo protests that if he had a woman such as Imogen, he would never pursue common prostitutes. Imogen is worried, saying that her husband "has forgot Britain" (line 113). Iachimo says Posthumus is forcing her to live a nun-like life while he is living a sexually promiscuous life on her money. He urges her to revenge and dedicates himself to her "sweet pleasure" (line 136), saying that unlike Posthumus, he will be loyal to her.
He tries to kiss Imogen, but this action prompts her to see Iachimo for what he is. She calls for Pisanio, and rejects Iachimo's advance, reproaching herself for listening to him. His motivation, she says, was not virtuous but self-seeking, to obtain her for himself. She says he has wronged Posthumus and that she will tell her father of his assault.
Iachimo rapidly changes direction, protesting that his aim was only to test her loyalty to Posthumus, and that he did so out of love to him. He begs her to forgive him, which she does.
Iachimo then asks Imogen to store a trunk containing expensive plate and jewels which he, Posthumus and some other friends have jointly bought as a gift for the Roman Emperor. She agrees, and as Posthumus has an interest in them, says she will keep them in her bedroom. Iachimo says he will have them sent to her just for tonight, as he must leave tomorrow. She tries to persuade Iachimo to stay, but he cannot. He suggests that she write Posthumus a letter tonight so that he can return with it tomorrow.
Iachimo's base nature is displayed in his use of animal imagery (lines 39-50). Apes, he says, would discriminate better than Posthumus does between Imogen and the imaginary "slut." He likens Posthumus to a predator which devours first the lamb (Imogen) but, still hungry, wants to feed on "garbage."
Iachimo's description of Posthumus in exile as "the Briton reveler" is calculated to upset Imogen and make her suspicious of her husband's loyalty. He uses images of rot, decay, disease and poison to excite disgust at Posthumus's supposed life of whoring: "Such boil'd stuff / As well might poison poison!" (line 126) is thought to refer to the sweating treatment given to people suffering from venereal disease. Note that he also tries to set Imogen against Posthumus by accusing Posthumus of going whoring on her money, a continuation the theme of buying and selling.
Iachimo's dedication of himself to Imogen's "sweet pleasure" may be taken to mean he will do what she wishes by way of helping her take revenge, or may be a seduction attempt. Probably, it carries both meanings. His mind constantly runs along animalistic and sexual tracks, debasing the love relationship between Posthumus and Imogen.
The rapidity with which Iachimo changes track once Imogen sees through him, and appeals to her naturally open nature, indicates his great skill in manipulating people. Imogen's willingness to forgive Iachimo contrasts markedly with her father's failure to forgive her for marrying against his wishes.
Cymbeline: Novel Summary: Act 1 Scene 7