In Britain during the time of Roman rule, gossip about King Cymbeline’s family strife swirls around court. Princess Imogen has secretly married Posthumus, a wealthy Roman orphan brought up by Cymbeline. The King, however, wanted Imogen to marry his stepson, Cloten, who is the only son of his second wife, the Queen. Imogen is Cymbeline’s sole heir because Cymbeline’s two sons, Guiderius and Arviragus, were stolen from their nursery as infants twenty years ago and never seen again. After discovering Imogen’s clandestine marriage, Cymbeline banished Posthumus and imprisoned Imogen, placing her under the Queen’s care.
Imogen and Posthumus (who has not yet left for his exile) pledge their fidelity to each other and exchange tokens of loyalty—Imogen gives Posthumus her mother’s ring, and he gives her a bracelet in return. Cymbeline discovers the lovers and flies into a fury. After Posthumus leaves, his servant Pisanio promises he will remain loyal to Imogen and serve her in Posthumus’ absence. The Queen has promised Imogen that she will be her ally, but Imogen suspects that she is up to no good. These suspicions prove true: the Queen has procured what she believes to be poison from the court doctor, Cornelius. She gives this to Pisanio, pretending that it is a restorative medicine. Nevertheless, the doctor reveals to the audience that he has given the Queen a strong sleeping medication instead of poison, since he doesn’t trust her.
In Italy, Posthumus takes up his exile as a guest of his father’s friend, Philario. At Philario’s house, he encounters the nobleman Iachimo, who bets Posthumus that he can make Imogen unfaithful. Posthumus bets his ring and his wife’s chastity against Iachimo’s gold. Iachimo goes to Britain and attempts to seduce Imogen with flattery, but his overtures anger Imogen, and Iachimo lies, saying that he was only testing her loyalty. Iachimo asks if he can safely keep a trunk filled with jewels in Imogen’s bedroom for safekeeping, and Imogen agrees. However, Iachimo hides himself in the trunk, popping out once Imogen is asleep. He notes details about the room—and the fact that Imogen has a mole on her left breast—and steals Posthumus’ bracelet, which he will use as “proof” that Imogen yielded to his sexual advances.
Shortly thereafter, Cloten tries to woo Imogen with music, but she rebuffs him, saying that she would prefer Posthumus’ “mean’st garment” to Cloten. Offended by her refusal, Cloten swears he will take revenge on Imogen. Meanwhile, Iachimo returns to Rome and delivers his “evidence” of Imogen’s infidelity to Posthumus. Reluctant at first to believe Iachimo, Posthumus eventually takes Iachimo’s “proof” as true. Posthumus gives up Imogen’s diamond ring to Iachimo and, like Cloten, swears he will take revenge on Imogen, before delivering a misogynistic soliloquy on the faults of women.
Back at the British court, the Roman ambassador Lucius asks Cymbeline for his tribute (money paid to the Emperor Augustus to guarantee peace). The Queen and Cloten urge Cymbeline to withhold the tribute money, since they believe that Britain should become independent of the Roman empire. Eventually, they persuade Cymbeline to refuse to pay the tribute. As a result, Lucius reluctantly informs Cymbeline that he will have to report this act of rebellion to the Emperor, who will now consider Cymbeline an enemy. War between Britain and Rome looms large, and Roman officials gather troops for an invasion. While Lucius will command the whole Roman army as its general, Iachimo will lead troops of Italian gentlemen, recruited by the Roman Senate.
As political relationships grow stormy, so, too, do romantic ones. Posthumus has sent a letter to his servant Pisanio, asking him to kill Imogen for her infidelity. Posthumus has also provided a letter for Imogen, asking her to meet him at Milford Haven in Wales. The letter is a trick: Posthumus hopes that Pisanio will kill Imogen while she’s away from court on her journey to supposedly meet him.
Meanwhile, in Wales, Belarius (under the alias Morgan) and his adopted sons Polydor and Cadwal live in a cave. The three men discuss the merits of living in nature versus living at court. The sons are eager to roam beyond the confines of their home in the wilderness, but Belarius warns them about the treachery of life at court. As an example, he explains that two scheming men lied to Cymbeline, saying that Belarius was allied with Rome instead of Britain, and Cymbeline banished him in turn. Belarius tells the boys to go ahead and hunt, and as soon as they are gone, he reveals the truth to the audience. Belarius was the one who stole the princes in their nursery twenty years ago as a way to get back at Cymbeline for his banishment—Polydor is actually Guiderius, and Cadwal is Arviragus. Despite the brothers’ rustic upbringing, Belarius fears that the princes’ noble nature is starting to reveal itself.
On the road to Milford Haven, Pisanio tells Imogen the truth: Posthumus wants her dead for her alleged infidelity. Heartbroken, Imogen asks Pisanio to kill her right then and there, but Pisanio refuses. He explains that he brought her on this journey to buy time and devise a plan: she can dress up in a man’s clothing and join Lucius as a servant to save her own life. Imogen agrees, and he leaves her with the medicine the Queen gave him in case she feels unwell. Back at court, Cymbeline has learned of Imogen’s absence, and Pisanio shows the King the letter from Posthumus asking Imogen to meet him at Milford Haven. After Cloten learns about Imogen’s location, he swears to disguise himself in Posthumus’ clothes. He plans to kill Posthumus, rape Imogen, and return her to court to make his revenge for her rejection complete.
In the meantime, Imogen has taken up male disguise, but, having lost her way, she is overcome with hunger and exhaustion. She stumbles upon Belarius, Guiderius, and Arviragus in the cave, and introduces herself as a boy, Fidele. They welcome Imogen and form a close bond. Feeling unwell, Imogen says she will rest and she takes the medicine Pisanio gave her while the men go off to hunt. While the men are hunting, they encounter Cloten in Posthumus’ clothes. Cloten insults the men, and Guiderius fights him, killing and beheading Cloten. Back at the cave, Arviragus discovers Fidele in a death-like trance and pronounces him dead. They lay his body next to the grave of their supposed mother, Euriphile (who was the princes’ nurse). They also lay Cloten’s decapitated body in Posthumus’ clothes next to Fidele. After the men sing a mourning song and leave, Imogen wakes up, believing she has found her dead husband’s body beside her.
Just then, Lucius and his troops stumble upon Imogen and the body. She presents herself as Fidele, the servant of the decapitated man, and she laments his death at the hands of murderous mountaineers. Lucius admires Fidele’s devotion to his seeming master, and takes Fidele into his service.
After Cloten’s death, Guiderius and Arviragus convince Belarius that they should take up arms against the Romans in defense of Britain. Meanwhile, Posthumus, who has heard rumors that Imogen has been murdered, feels remorseful and longs to be killed in the war as penance. He switches his uniform to belong to the side that is losing (currently the Romans). The Britons win the battle, thanks in part to the efforts of Belarius, Guiderius, and Arviragus (who saved Cymbeline’s life during the fighting). British captains spy Posthumus in his Roman uniform and capture him. Not recognizing his son-in-law, Cymbeline sentences Posthumus to prison to await execution.
While in the jail, Posthumus falls asleep and has a dream vision of Jupiter descending on an eagle. The ghosts of his father (Sicilius Leonatus), his Mother, and his Brothers appear, giving him encouragement and pleading on his behalf to Jupiter. Jupiter assures Posthumus that everything will work out for the best and leaves a tablet inscribed with a puzzling prophecy on the sleeping Posthumus’ chest. When Posthumus wakes up, he is taken before Cymbeline.
Cymbeline knights the men he believes are Morgan, Polydor, and Cadwal for their bravery in battle. The doctor reports that the Queen, sick with grief over her missing son, has died. Before succumbing to a fever, she made a deathbed confession of her treachery—that she attempted to poison Pisanio and that she truthfully hated (and tried to slowly kill) the King, wanting him dead so that she and Cloten could rule. Soon after Cymbeline learns this truth, Roman soldiers—including Iachimo, Lucius, and Fidele—are brought before him. Lucius begs for Fidele’s life to be spared, and he expects Fidele to beg for his life, too. Instead, Fidele only requests to know where Iachimo got the diamond ring on his finger, which originally belonged to Imogen.
Iachimo confesses the truth about his bet, and Posthumus flies into a rage. Fidele tries to calm Posthumus, but Posthumus knocks Fidele down. At that moment, Pisanio tries to help his mistress and he reveals that Fidele is actually Imogen in disguise. The couple reunites, and Imogen asks for Cymbeline’s blessing of their marriage, which he freely gives. When Guiderius confesses to killing Cloten, Belarius pleads for his son’s life and reveals the truth about Guiderius and Arviragus’ identity. Lucius’ Soothsayer deciphers the tablet that Jupiter left on the sleeping Posthumus’ chest during the dream vision: it predicts a reconciliation between Posthumus and Imogen, the recovery of Cymbeline’s sons, and peace and prosperity in Britain. Overjoyed, Cymbeline pardons everyone and says he will resume paying the tribute to Rome. He orders that peace be proclaimed, and everyone leaves to go to London and give thanks at Jupiter’s temple.