Above: Posthumus, Philario, a Frenchman and Iachimo in a tense discussion about Imogen's faithfulness
The cast relates King Cymbeline’s family history, and the disappearance of his son 20 years before. During this preamble, the audience discovers that Posthumus Leonatus has secretly wed Cymbeline’s daughter Imogen, and has been banished because he is not thought worthy to be her husband. The Queen, Imogen’s stepmother, professes to help them, but actually works to ensure Posthumus' removal from court. The Queen wishes for her son, Cloten, to marry Imogen and become King. Cloten, a laughing-stock among the lords, attempts to woo Imogen but she rejects him. The Queen asks a doctor, Cornelius, for a poison, intending to kill Imogen. She gives it to Pisanio to take to Imogen, telling him it is a rejuvenating cordial, and suggests that she should drink it if she feels ill. Unbeknownst to the Queen, the doctor distrusting her, has substituted a potion that instills only a harmless temporary death-like state.
Posthumus arrives in Rome, where he meets Iachimo, who challenges Posthumus’ claim that Imogen is the most faithful woman in the world. They make a wager, Iachimo betting he can seduce Imogen.
When Iachimo arrives at Cymbeline’s court, he presses his affection on Imogen, 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 Imogen has been unfaithful, and he gives Iachimo Imogen's ring, condemning her and all women.
Caius Lucius arrives at court from Rome demanding the British tribute, but leaves without success. Posthumus writes to his servant, Pisanio, telling him of Imogen's unfaithfulness, and that he must murder her. Imogen also receives a letter from Posthumus, telling her he will meet her in Wales—actually only providing Pisanio an opportunity to kill her. Imogen insists on going there.
Pisanio knows Posthumus has been misled, and that Imogen 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 Imogen the Queen's potion, still thinking it to be a rejuvenating cordial. Pisanio returns to England, and Cloten forces from him Imogen's whereabouts; then, dressed in Posthumus' clothes, Cloten follows her to Wales.
In the Welsh mountains, Belarius sends Guiderius out hunting, reflecting on his true origin as son of Cymbeline, and on his own former position in court, from where he stole the boy in revenge for his unjust banishment. Imogen gets lost, and takes refuge in their cave. Guiderius meets her, and immediately forms a strong attachment 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 Imogen. 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 herself Fidele.
The Roman army advances, and Guidarius persuades Belarius to let him fight. Posthumus has come as part of the Roman forces, but decides to fight for Britain, in reparation for what he has done to Imogen. Iachimo is also in the Roman army, regretting what he has done. In the battle, Cymbeline is taken but rescued by Belarius, Guidarius and 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 Guidarius. 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 Imogen. The disappearance of Cloten is explained, and Cymbeline reluctantly condemns Guiderius, but he is pardoned when Belarius reveals the true status of his son. Iachimo is forgiven, the prophesy is explained, and Cymbeline makes peace with Rome.