An excerpt from the beginning of the first chapter:

During the time of Augustus Caesar, Emperor of Rome, there reigned in England (which was then called Britain) a king whose name was Cymbeline.

Cymbeline's first wife died when his three children, two sons and a daughter, were very young. Imogen, the eldest of these children, was brought up in her father's court; but by a strange chance the two sons of Cymbeline were stolen out of their nursery when the eldest was but three years old, and the youngest quite an infant; and Cymbeline could never discover what had become of them, or by whom they were taken away.

Cymbeline was twice married: his second wife was a wicked plotting woman, and a cruel step-mother to Imogen, Cymbeline's daughter by his first wife.

The queen, though she hated Imogen, yet wished her to marry a son of her own by a former husband (she also having been twice married), for by this means she hoped upon the death of Cymbeline to place the crown of Britain upon the head of her son Cloten ; for she knew that, if the king's sons were not found, the princess Imogen must be the king's heir. But this design was prevented by Imogen herself, who married without the consent or even knowledge of her father or the queen.

Posthumus, that was the name of Imogen's husband, was the best scholar and most accomplished gentleman of that age. His father died fighting in the wars for Cymbeline, and soon after his birth his mother died also for grief at the loss of her husband.
Cymbeline, pitying the helpless state of this orphan, took Posthumus (the name Cymbeline gave him, because he was born after his father's death), and educated him in his own court.

Imogen and Posthumus were both taught by the same masters, and were play-fellows from their infancy: they loved each other tenderly when they were children, and their affection continuing to increase with their years, when they grew up they privately married.

The disappointed queen soon learnt this secret, for she kept spies constantly to watch upon the actions of her daughter-in-law, and she immediately told the king of the marriage of Imogen with Posthumus.

Nothing could exceed the wrath of Cymbeline when he heard that his daughter had been so forgetful of her high dignity as to marry a subject. He commanded Posthumus to leave Britain, and banished him from his native country for ever.

The queen, who pretended to pity Imogen for the grief she suffered at losing her husband, offered to procure them a private meeting before Posthumus set out on his journey to Rome, which place he had chosen for his residence in his banishment: this seeming kindness she showed, the better to succeed in her future designs in regard to her son Cloten; for she meant to persuade Imogen, when her husband was gone, that her marriage was not lawful, being contracted without the consent of the king.