The scene is set in Imogen's bedroom. Iachimo's trunk is placed in it. After she goes to sleep, Iachimo emerges from the trunk. He is tempted by her beauty, but resists the urge to touch and kiss her. His plan is to note details of the room and identifying marks on her body, in order to convince Posthumus that he has succeeded in seducing her. He removes her bracelet, the parting gift from her husband, which he says will help in the "madding" of Posthumus (line 37). He also notes a mole on her left breast. When he feels he has enough evidence, he gets back into the trunk.
The atmosphere of sexual menace in this scene is palpable. Iachimo, leaning over the sleeping and defenseless Imogen and noting the intimate details of her body, is committing a kind of rape, a psychological violation of the marriage between her and Posthumus. This notion is reinforced by the book she has been reading (probably Ovid's Metamorphoses), which tells of treason and rape. Iachimo appears to liken himself to a raven (line 49), a continuation of the predator and prey imagery that runs through the play. His line "Though this is a heavenly angel, hell is here" (line 50) leaves no doubt as to the depraved state of Iachimo's soul as compared with that of Imogen.
Cymbeline: Novel Summary: Act 2 Scene 2