Posthumus has received from Pisanio a bloodied cloth, indicating Imogen’s death. He is sad that she is dead and for a small transgression while other people get away with worse crimes. He wishes that he had been killed instead for one of his transgressions. He decides to make amends--however futile the gesture. Though he has come to Britain arrayed as a Roman and to fight as one, he will dress himself like a British peasant and die while fighting the Romans (which he thinks will give him a better chance to die).
Having been bested by a British peasant (Posthumus), Jachimo reflects upon his guilt, the guilt of having besmirched an honorable British lady as the cause of his ignoble defeat. It’s either that or even a British peasant is the mightier soldier than a Roman noble in which case the Romans would have no chance of winning the war. (For if a British peasant can manhandle a Roman noble what might a British noble be capable of?) Presently, Cymbeline is being taken captive by the Romans only to be rescued by Morton, Polydore, Cadwal, and Posthumus. The Romans, including Lucius, Jachimo, and Fidele, retreat as certain Roman victory has turned into unmanageable chaos.
A British Lord, who had fled the scene, inquires Posthumus as to how a certain British defeat had been averted and turned into victory. Posthumus explains that the credit goes to a white bearded old man and two youngsters who stood their ground and repelled the Romans which inspired the mass of fleeing British cowards to turn back and fight. When the Lord continues to express his amazement, Posthumus disparages him for having so much to say and so little to offer in deeds. Alone, Posthumus vents his dissatisfaction with the state of the world that would spare cowards of their shame by depriving them of their lives while it would spare Posthumus and thereby torture him with his guilt concerning Imogen’s death. Determined to die one way or the other, Posthumus decides to put himself at a disadvantage by changing his garb to that of a Roman’s. By and by, he his taken captive by the British.
Imprisoned, Posthumus takes comfort in the fact that death (by execution) will soon free him from his guilt with regards Imogen’s murder. He falls asleep and has a dream wherein his dead father, mother, and two brothers appeal to Jupiter for Posthumus’ welfare. Jupiter answers their appeal by chiding them for thinking that Jupiter has unjustly made Posthumus suffer. Jupiter assures them that Posthumus will eventually find happiness and prosperity and leaves a book for Posthumus to read that proclaims his salvation. Posthumus wakes up, regretting that doing so has deprived him of his dream and his long-lost family. He sees a book lying on his chest and reads. Its contents mirror so much of what he has experienced in life that he decides to keep it. By and by, the jailers summon Posthumus to his execution which Posthumus is only too eager to attend. He must first receive his official sentence from the the King, however, to which end he is unchained.
Cymbeline is praising Morton, Polydore, and Cadwal for their heroic service rendered Britain while regretting the absence of the fourth man in rags who had rendered a service equally heroic when Cornelius, the physician, appears to report some stunning news. The Queen has died but not before confessing that she had never loved the King. Indeed her ambition was to kill both the King and his daughter, so that her son would assume the throne. The Queen’s Ladies confirm Cornelius’ words. By and by, Roman prisoners are brought to the court. Conceding defeat, Caius Lucius begs the King to grant him one wish: that the King spare Fidele’s life, citing Fidele’s British heritage and the fine service he has rendered Lucius. Cymbeline agrees to do this but not because Lucius has requested it but because the King has taken a liking to the boy though he could say not why. Thus Cymbeline offers to grant Fidele a wish of his own. Lucius thinks that Fidele might plead for his--Lucius’s--life, but that is not it at all. Jachimo is summoned forth and is demanded to explain how he had got in possession of his gold ring. Though doing it in the most roundabout manner that exasperates the King, Jachimo reveals all: of how he had wagered Posthumus with regards Imogen’s faith and chastity, and of how he had fooled Posthumus into thinking that he--Jachimo--had won the wager by presenting evidence that made it seem as if Jachimo had slept with Imogen, winning him the gold ring. At this point, Posthumus steps forth, denounces Jachimo, and then identifying himself as Posthumus he denounces himself, infoming the King he--Posthumus--had Imogen killed, believing her to be unfaithful. However, it isn’t long before Fidele reveals himself to be Imogen thus reuniting Imogen and Posthumus. Concomitantly, Morton, Polydore, and Cadwal realize that Imogen was the Fidele whom they had buried, thinking he was dead.
Thus everything seems to be in good order when the King’s mention of Cloten’s mysterious absence prompts Pisanio to mention Cloten’s perusal of Posthumus’ letter, of how that had inspired Cloten to put on Posthumus’ garment and ride out to Milford-Haven where he meant to kill Posthumus and ravish Imogen. This in turn prompts Polydore to confess that it was he who had killed Cloten. The King begs Polydore to recant his statement, and when he doesn’t the King has no choice but to condemn Polydore to death. Morton steps in at this point to reveal the most fantastic new of all. First of all he reveals to the King that he--Morton--is really Belarius, the man the King had banished many years ago. He then reveals that Polydore and Cadwal are in fact Guiderius and Arviragus the King’s abducted sons who were raised by Belarius and the boys’ nurse, Euriphile, who had eventually married Belarius. By and by, Guiderius and Arviragus recognize Posthumus as the fourth man in the rags who had fought by their side and embrace him as their brother. Imogen likewise embraces her brothers, marveling at the fact that she had regarded them as brothers when she was Fidele and they Polydore and Cadwal.
All is well in Britain. As for the Roman prisoners, all are granted mercy as Cymbeline follows the example of his son-in-law Posthumus who has forgiven Jachimo. Additionally, Cymbeline agrees to pay Rome its tribute thereby reversing the policy that was set in motion by the Queen.