Cymbeline, king of Britain, is angry because his daughter, Imogen, has secretly married Posthumus, a poor but worthy gentleman. Cymbeline's evil second wife, Imogen's stepmother, would rather Imogen had married her own stepbrother, Cloten, the queen's son by an earlier marriage. The king banishes Posthumus, who goes to Rome where he meets the crafty Iachimo, who claims that no woman can be virtuous and wagers Posthumus that he can seduce his wife, Imogen. Iachimo goes to England but sees at once that she can never be won; so he hides in a chest which he has asked Imogen to safely keep in her bedroom.
That night, after Imogen is asleep, Iachimo steals out of the chest, takes careful notes of her room and her exposed person, and steals the bracelet on her arm, a bracelet her husband had given her. With this he returns to Posthumus, who is frantic over the apparently incontestable evidence of his wife's inconstancy.
In despair, Posthumus sends orders to his faithful servant, Pisanio, to kill Imogen. The good Pisanio, instead, persuades her to leave the court and escape death as well as the hatred of the queen and the advances of her son, Cloten. Disguised as a page, she comes to the cave where Belarius, a banished nobleman, lives as a peasant. Belarius is raising as his own sons the two children of Cymbeline whom he had stolen from their nursery twenty years before. They pity the solitary little page, for whom they feel an unaccountable affection.
Cloten, dressed in Posthumus's clothing, soon comes in pursuit of Imogen. Belarius has one of the sons cut off the prince's head. Meanwhile, Imogen, to calm herself, drinks medicine that Pisanio, thinking it a wonderful cordial, had innocently received from the queen. However, the queen intended to give Pisanio a deadly poison. Thus the brothers are horrified to find their beloved page apparently dead. Taking her tenderly to the forest, they lay her next to Cloten's body. Since the drug was really only a sleeping potion, Imogen soon wakes and, seeing the headless body she believes to be her husband, falls in a faint. The Roman ambassador approaches as she is recovering, and she takes service with him as a page.
Meanwhile, Cymbeline is preparing for war with Rome, and the noble brothers and their "father," Belarius, join the king's forces. In battle the three men rescue Cymbeline from the Romans and capture the Roman ambassador and his page. Posthumus is in despair over Imogen's supposed death and disguises himself as a Roman prisoner of war so Cymbeline will have him put to death.
The king learns that his wife has died and has confessed her treachery. Imogen then forces Iachimo to confess his treachery to her and Posthumus. She then reveals her identity to her surprised husband, who is quickly recognized and released. Next, Belarius reveals the two princes to their father, Cybeline, who pardons the old man and the Roman ambassador and makes peace with Rome.