It is unclear when Cymbeline was first written, though most scholars consider it was probably produced by 1611 and was included in the First Folio, where it was classified as a tragedy. Other critics consider Cymbeline more of a romance or comedy, and it draws upon the same familiar themes of jealous love and innocence that are highlighted in many Shakespearean works.
The play opens when the Roman Empire's king of Britain Cymbeline discovers his daughter Imogen has eloped with her lover Posthumus Leonatus. This is a great surprise, as Posthumus was previously considered an honorable member of court. Cymbeline still grieves the loss of his two sons who were stolen by Belarius, a traitorous criminal who was exiled and made off with Guiderius and Arviargus.
The marriage seems to have been serious to Imogen as she exchanged jewelry with Posthumus, but her father does not share her sentiment and banishes her new husband. His reasoning is that Imogen must produce an heir with a full royal blood line.
Cymbeline's Queen aspires to have Imogen marry her first born son Cloten, but also has deadly aspirations that include killing both Imogen and Cymbeline with poison. This poison is passed along to the servant Pisanio while the distraught Imogen, separated from Posthumus, isolates herself in her chamber away from Cloten.
Posthumus is exiled to Italy where he meets Iachimo. Iachimo would like to play a game where he seduces Imogen and then gives evidence of his victory to Posthumus, who insists on Imogen's chastity. A bet is made wherein a duel with be forthcoming if Iachimo loses, which he does after trying to seduce Imogen. Iachimo sneakily hides in her room and steals a bracelet and glimpses her naked body and holds all this information as false evidence of his seduction.
Posthumus orders Pistanio to murder Imogen but he refuses, and instead tells Imogen to dress as a boy in disguise. Not knowing the poison from the Queen is that, he gives it to her as anxiety relief. She takes the false name of Fidele.
Cymbeline is in trouble for not showing respect to the Roman Caius Lucius and now is at risk for invasion from Rome. Cloten learns of the plan Posthumus made to murder Imogen and decides to interject, steal her, and rape and marry her. But Imogen, now Fidele, falls sick right outside the cave of Belarius and so runs into her brothers Buiderius and Arviagus. None realize their origin, though the brothers like Fidele and outside behead Cloten.
Believing Cloten is Posthumus , Imogen is horrified and grieves and her display of emotion lands Fidele a job as Lucius' pageboy. The Queen grieves over her lost Cloten, and Posthumus enlists to help the Rome in their invasion of Great Britain. Belarius and the two lost sons enlist to help Britain, and though Cymbeline does not recognize them he sees their valor in battle.
The Queen dies and on her deathbed confesses to all her wrongdoings. Imogen and Posthumus are reunited after her identity is revealed to him. Belarius also makes a shocking confession and reveals that Guiderius and Arviragus are Cymbeline's lost sons.
Their return to the line of succession entitle Imogen to marry Posthumus, and this revelation causes Cymbeline great joy. He blames his second wife the Queen for many grievances, and Britain and Rome are brought together for a great feast where they celebrate the new peace between their countries.