Back at Cymbeline's court, the disappearance of the Queen's son, Cloten, has stricken her with a wasting fever. Cymbeline threatens Pisanio with torture in an attempt to find out to where Imogen has fled, but the Roman invasion of Britain intervenes, and Cymbeline must prepare his army to meet the new threat. Meanwhile, Guiderius, Arviragus, and Belarius hear armies moving through the wilderness; Belarius wants to lie low, since he is afraid that some of the Britons may recognize him from his days at court, but his adoptive sons are eager to fight, and they insist that they go down to assist Cymbeline's forces.
Posthumus returns to Britain, having been conscripted into the Roman forces as an Italian resident. He has received a bloody handkerchief from Pisanio, ostensibly a token of Imogen's death, and he is overcome with remorse, resolving that "'Tis enough / That, Britain, I have killed thy mistress; peace, / I'll give no wound to thee (V.i.19-21)." He takes off his Roman uniform and dresses himself as a British peasant for the battle.
During the fight, Iachimo, fighting on the side of the Romans, loses his sword in a duel with the disguised Posthumus. Left alone, he expresses his remorse for having lied about Imogen's faithlessness. Meanwhile, the battle goes badly for the British until the sudden arrival of Belarius, Guiderius, and Arviragus, who save Cymbeline from capture and turn the tide. The Romans are beaten; Caius Lucius is taken prisoner--along with Posthumus, who, although he fought for the victorious Britons, is trying to punish himself for his supposed murder of Imogen, and he has quickly changed back into Roman garb in order to be taken prisoner. He is thrown into a British stockade and, desiring death, falls asleep.
While he sleeps, a collection of spirits ascends from the netherworld and gathers around him. They are Posthumus's dead ancestors, and they plead with Jupiter, the king of the gods, to take pity on their descendant and restore his fortunes. After a time, Jupiter himself arrives from the heavens, surrounded in thunder and lightning and riding on the back of an eagle. He berates the spirits for troubling him, but he grudgingly agrees to bring about happiness for Posthumus. Then, all the supernatural creatures depart, and Posthumus awakens, feeling strangely refreshed, and finds a written oracle on the ground beside him, which he is unable to interpret. The jailer comes to take him to be hanged, but then a messenger arrives, summoning Posthumus to stand before Cymbeline.
With Cloten already dead, now his wicked mother's time has come, as well; her reported illness is the last we hear of her before her death in V.v. The action of the play now draws all the characters into the swirl of battle, including Guiderius and Arviragus, who see the war as an excellent opportunity to escape from the prison of their wilderness exile, and they easily overcome their father's anxious objections. Iachimo and Posthumus also reappear, having been conveniently drafted out of Renaissance Italy and into the Roman army. Iachimo feels his first pangs of remorse, suggesting, much to the audience's satisfaction, that he was not so bad after all and that Shakespeare means to spare his life. Posthumus is also remorseful, but his speech before the battle is unlikely to draw much sympathy: Although he has-- thankfully--realized the immorality of murdering his wife, he persists in believing that she was unfaithful to him. Thus, while he has recovered his sense of perspective (too late, or so he thinks), he has yet to display the trust, gentleness, and soundness of judgment that would make him worthy of such an exemplary wife as Imogen.
The battle occurs in a scene heavy with stage directions and remarkably thin on dialogue compared with other Shakespearean battles. Scant, too, are the trumpets ("Alarums") that one normally finds in such scenes; their absence may be a function of the fact that Shakespeare was writing for a smaller, indoor stage (the Blackfriars Theater) rather than the famous and massive Globe Theater. Guiderius, Arviragus, and Belarius's crucial role in these scenes comes as no surprise, and their intervention here operates as a necessary preliminary to the general reconciliation in the play's last scene.