Cymbeline brings Guiderius, Arviragus, and Belarius before him to reward them for their valor in battle. He regrets that the unknown peasant who fought so well for Britain (who is, of course, Posthumus) cannot be found, and he then proceeds to knight Belarius and the two young men (who are his own sons, though he does not know it) in gratitude for their service.
Just then, Cornelius comes in, bringing word that the Queen has died of her fever. Before she died, he reports, she confessed that she never loved Cymbeline and planned to gradually poison him so that the crown would devolve upon her son, Cloten. The king, amazed, says that she managed to deceive him completely, and he attributes her success in this to her great beauty.
The Roman prisoners, including Caius Lucius, Iachimo, and Posthumus, with Imogen (still disguised as the boy Fidele) following at the rear, are all brought in together. The Roman general asks that Cymbeline treat them mercifully--and asks especially that his servant, a British boy (who is, of course, the disguised Imogen), be ransomed and freed. Imogen is then brought before her father, who does not recognize her but orders her freed and even offers her any privilege within his power to grant. She asks to speak with him in private, and father and daughter separate themselves from the rest of the company. When they return, Imogen asks Iachimo to step forward, and she demands to know where he got the ring that encircles his finger (the audience knows that Imogen gave the ring to Posthumus and that Posthumus lost it to Iachimo in the wager). Iachimo, feeling pangs of remorse, confesses how he used trickery to win the bet with Posthumus, describing his entire scheme to gain entrance to Imogen's bedroom. Hearing the story, Posthumus attempts to assault Iachimo, but Imogen hastily reveals her true identity, stripping off her boy's disguise, and the reunited couple embraces.
Through dialogue, the characters piece together the story of how Imogen came to the cave, how she only appeared dead after taking the Queen's potion, and how Cloten met actual death. Cymbeline declares that Guiderius must die for killing a prince, but Belarius hastily reveals himself as the banished courtier and tells the king that Guiderius and Arviragus are the sons that were stolen from him long ago. Cymbeline, overcome with happiness, forgives Belarius and welcomes him back to court; meanwhile, Iachimo offers his life to Posthumus as payment for his sins, but Posthumus graciously forgives him. Caius Lucius's Soothsayer comes forward and interprets the prophecy that Posthumus found beside him that morning (left by Zeus), which is revealed to refer to the reunion of Imogen with her husband and the return of Cymbeline's two sons. Caught up in the abiding joyful spirit, the king promises to free the Romans, to allow them to return home unpunished, and even to resume the tribute, which was the issue over which the war was fought in the first place. Rejoicing, the entire company exits together to make a great feast and offer sacrifices to the gods.
This final scene, with its multiple revelations and subsequent happy ending, is at once the culmination of the action and the finest part of the play, the best display of Shakespeare's consummate skill as a dramatist. The action properly begins with the calling forth of Imogen, still clad in boy's clothes, and the audience anticipates the removal of her disguise and a hasty denouement. Instead, she spots the ring on Iachimo's finger and begins to interrogate him, thus, giving him the opportunity to make the first revelation. The Italian's flair for the dramatic serves him as well in the role of repentant sinner as it did in his previous incarnation as a deceptive villain, and the ensuing speech imbues the scene with much color. Confessing his crimes to Cymbeline's court, he deliberately draws out his story slowly, building the suspense: The impatient Cymbeline pleads, "I stand on fire / Come to the matter (V.v.168-69)," but Iachimo, who knows that this may be the last performance of his life, will not be hurried. Finally, he reaches the end, and still Imogen does not reveal herself, giving Posthumus a chance to come forward and finally (if melodramatically) recognize the virtue of his wife and the extent of his folly. His speech, with its piteous cries of "O Imogen, / My queen, my life, my wife! O Imogen, / Imogen, Imogen! (V.v.225-27)," goes a little way toward restoring the audience's sympathy for him--and it gives way to the comic moment when Imogen goes to embrace him in forgiveness, and he thrusts her aside, not recognizing her. (Her disguise is indeed impressive!)
Once Imogen's true identity is revealed, the rest of the revelations spill out quickly. Pisanio and Cornelius explain about the poison--now Belarius, Arviragus, and Guiderius understand how it was that "Fidele" came back to life-- and then Guiderius confesses to killing Cloten. Cymbeline (who, despite being free of the Queen's influence, remains somewhat emotionally dense) now threatens to execute Guiderius--this, in turn, prompts the revelation of Guiderius and Arviragus' true identities. And now such happiness reigns that Cymbeline cannot help but pardon Belarius for abducting his sons (a rather serious crime, one might think): "Thou art my brother," the king tells the banished lord; "so we'll hold thee ever (V.v.399)." With this example, Posthumus must pardon Iachimo, and Cymbeline, in turn, must free the Romans--whose general, it may be added, is one of the most honorable and decent men in the entire play.