Cymbeline, accompanied by the Queen and Cloten, bids farewell to Caius Lucius. The king then asks to see Imogen and sends a messenger to fetch her, but the messenger returns saying that the door to her bedroom is locked, and she has not been seen in days. Suddenly worried, Cymbeline goes to see for himself, and Cloten follows. After a moment, the Queen's son returns, with word of Imogen's flight. The Queen goes to comfort Cymbeline, and Cloten is left alone to fume and plot revenge on Imogen and Posthumus. Pisanio comes in, returning from Milford Haven, and Cloten accosts him, demanding to know where the princess has gone. Pisanio, deciding that his mistress has had enough time to make her getaway, sends Cloten to the sea coast on what he knows will be a wild goose chase. The foolish prince, convinced that he will catch Imogen and Posthumus, takes one of Posthumus's garments with him--the same garments Imogen claimed to prefer to Iachimo--planning to kill his rival and then rape Imogen while wearing Posthumus's clothes.
Meanwhile, Imogen, disguised as a boy, has become lost in the Welsh wilderness. She comes upon the cave where Guiderius, Arviragus, and Belarius live, and Imogen goes in to find shelter. Shortly afterward, the three men come home from a day of hunting, and they find her there, eating their food. She apologizes, offers to pay for the meat, and introduces herself as "Fidele." Guiderius and Arviragus, unaware that the boy Fidele is actually their sister, nonetheless feel a strange kinship with their guest, and Imogen reciprocates the feeling.
A Roman army under Caius Lucius makes ready to sail for Britain, while Cloten arrives at Milford Haven. Imogen, meanwhile, has fallen ill, and while her hosts go out to hunt, she takes the potion that Pisanio gave her, believing it to be medicine. In the forest, Cloten, dressed in Posthumus's clothing, encounters Guiderius, Arviragus, and Belarius, and he rudely challenges them to fight; Guiderius duels with the prince and kills him, cutting off his head. Belarius recognizes the dead prince from his days at court, and he panics, but his sons are elated, and Arviragus goes to wake "Fidele"--only to find the disguised Imogen seemingly dead. Dismayed and grief-stricken, Belarius and his adoptive sons lay her body in the woods, singing a prayer over her, and then depart, after setting Cloten's headless body down beside her.
After a time, Imogen awakes, and seeing the headless corpse dressed in Posthumus's clothes, assumes that it is her husband, dead. Realizing that the "medicine" she drank was a sleeping potion and believing Pisanio to have given it to her knowingly, she now thinks that the servant must also be responsible for killing Posthumus. Stricken with grief, she lays herself atop Cloten's body. Meanwhile, the Roman army has landed, and Caius Lucius and his men come upon Imogen and Cloten. At first, they think that both of them are dead, but Imogen arises, says that her name is Fidele, and offers herself as a servant to the Roman commander. Caius Lucius, believing her to be a young man, accepts her offer and employs her as his page.
Up until this point in the play, Cloten's stupidity is so pathetic that the audience may be inclined to feel some sympathy for the hapless prince. But because Shakespeare plans to kill him--and bloodily--he now sets about alienating us entirely from the Queen's son. Indeed, as soon as we learn of Cloten's bizarrely vicious and perverse plan, all sympathy vanishes: He says, "with [Posthumus's] suit upon my back will I ravish her, first kill him, and in her eyes (III.v.135-6)." Here, truly, is a character that only a mother could love.
Meanwhile, in Belarius's cave, Cymbeline's children are finally united; and if the audience has forgotten that Imogen, Guiderius, and Arviragus are all siblings, we are soon enough reminded by the three young people's strong feelings of kinship and by Imogen's regretful comment, "Would it had been so that they / Had been my father's sons (III.vi.75-76)!" She says this not only because of her sisterly feelings toward the youths but also because--as she notes to the audience--if the boys had been her father's sons, then Cymbeline would not have forbidden her marriage to Posthumus, for she would not have been heir to the throne and, thus, would not have been obliged to marry royalty.