Imogen, daughter of Cymbeline, King of Britain, has secretly married Leonatus Posthumus, a 'poor but very worthy gentleman'. The queen, Imogen's stepmother, determined that her clownish son Cloten shall marry Imogen, reveals the secret marriage to the king, who banishes Posthumus. In Rome, Posthumus boasts of Imogen's virtue and makes a wager with Iachimo that if he can seduce Imogen he shall have a diamond ring that Imogen had given him. Iachimo is repulsed by Imogen, but by hiding in her bedchamber he observes details of Imogen's room and her body which persuade Posthumus of her infidelity, and he receives the ring. Posthumus writes to his servant Pisanio directing him to kill Imogen; but Pisanio instead provides her with a male disguise, sending a bloody cloth to Posthumus to deceive him that the deed is done.
Under the name of Fidele Imogen becomes a page to Bellarius and the two lost sons of Cymbeline, Guiderius and Arviragus, living in a cave in Wales. Fidele sickens and is found as dead by the brothers, who speak the dirge ' Fear no more the heat of the sun'. Left alone she revives, only to discover at her side the headless corpse of Cloten which she believes, because of his borrowed garments, to be that of her husband Posthumus. A Roman army invades Britain; Imogen falls into the hands of the general Lucius and becomes his page. The Britons defeat the Romans, thanks to the superhuman valour in a narrow lane of Bellarius and his two sons aided by the disguised Posthumus. However, Posthumus, pretending to be a Roman, is subsequently taken prisoner and has a vision in gaol of his family and Jupiter, who leaves a prophetic document with him.
Lucius pleads with Cymbeline for the life of Fidele / Imogen: moved by something familiar in her appearance, he spares her life and grants her a favour. She asks that Iachimo be forced to tell how he came by the ring he wears. Posthumus, learning from this confession that his wife is innocent but believing her dead, is in despair till Imogen reveals herself. The king's joy at recovering his daughter is intensified when Bellarius restores to him his two lost sons, and the scene ends in a general reconciliation. Posthumus's words to Imogen on being reconciled with her, 'Hang there like fruit, my soul, Till the tree die!' were described by Tennyson as 'the tenderest lines in Shakespeare.'