Two gentlemen discuss King Cymbeline's family, and the disappearance of his two young children 20 years before. Posthumus, in love with Cymbeline's daughter Innogen, is banished because he is not thought worthy to be her husband. The Queen, Innogen's stepmother, professes to help them, but actually works to ensure his removal.
The Queen's son Cloten, a laughing-stock among the lords, is also in love with Innogen. He attempts to woo her but she rejects him. The Queen asks a doctor, Cornelius, for a poison, intending to kill Innogen, and gives it to Pisanio to take to her, telling hiim it is a rejuvenating cordial. The doctor, distrusting the Queen, substitutes a potion that instils only a harmless temporary death-like state.
Posthumus arrives in Rome, where he meets Iachimo, who challenges Posthumus' claim that Innogen is the most faithful woman in the world. They make a wager, Iachimo betting he can seduce Innogen. He arrives at Cymbeline's court, and presses his affection on her, but she rejects him. He then explains that this was only a stratagem to confirm her reputed worthiness, and she forgives him.
She agrees to give safe keeping to a large trunk of Iachimo's valuables that night. Iachimo hides inside the trunk, and while she is asleep he emerges from it to note details of her room and her body, and steals her bracelet. Back in Rome, this evidence convinces Posthumus that Innogen has been unfaithful, and he gives Iachimo her ring, condemning her and all women. Lucius arrives at Cymbeline's court from Rome demanding the British tribute, but leaves without success.
Posthumus writes to his servant, Pisanio, telling him of Innogen's unfaithfulness, and that he must murder her. Innogen also receives a letter from Posthumus, telling her he will meet her in Wales - actually only providing Pisanio an opportunity to kill her. Innogen insists on going there. Pisanio knows Posthumus has been misled, and that Innogen is innocent, so when they arrive in Wales he shows his master's letter to her. He convinces her to disguise herself as a man, and find service with Lucius, also now in Wales, so that she may be near Posthumus.
He gives Innogen the Queen's potion, still thinking it to be a rejuvenating cordial. Pisanio returns to England, and Cloten forces from him Innogen's whereabouts; then, dressed in Posthumus' clothes, Cloten follows her to Wales. In the Welsh mountains, Belarius sends Guiderius and Arviragus out hunting, reflecting on their true origin as sons of Cymbeline and on his own former position in court, from where he stole them in revenge for his unjust banishment. Innogen gets lost, and takes refuge in their cave. The sons meet her, and immediately are strongly attracted to her.
She feels unwell, and while the men are out she takes Pisanio's potion, falling into a death-like sleep. Guiderius meets Cloten; they quarrel, fight, and Guiderius beheads Cloten, whom Belarius later recognizes. They return to the cave, and find Innogen. Thinking her dead, they lay her on the ground in a burial ceremony, and place Cloten's headless body beside her.
When she wakes, she sees Posthumus' garments and assumes it is him. She faints across the body, and is found there by Lucius, who takes her into his service as a page. She calls hereself Fidele. The Roman army advances, and Belarius' sons persuade him to let them fight. Posthumus has come as part of the Roman forces, but decides to fight for Britain, in reparation for what he has done to Innogen. Iachimo is also in the Roman army, regretting what he has done. In the battle, Cymbeline is taken but rescued by Belarius and his sons along with Posthumus, and the British emerge as victors.
Posthumus puts on his Roman clothes and is captured; imprisoned, he looks forward to death. While sleeping, he is visited by the spirits of his dead family and by Jupiter, who leaves him a tablet containing a prophesy. Cymbeline knights Belarius and his sons. He learns that the Queen is dead and has confessed her wrong-doing. Lucius commends Fidele to Cymbeline, who grants her any request.
She asks for the ring she sees Iachimo wearing, and he confesses his villainy. Posthumus then reveals himself, and Fidele reveals herself to be Innogen. The disappearance of Cloten is explained, and Cymbeline reluctantly condemns Guiderius, but he is pardoned when Belarius reveals the true status of his sons. Iachimo is forgiven, the prophesy is explained, and Cymbeline makes peace with Rome.