Posthumus secretly married a friend from childhood, Imogen. However Posthumus was very poor and has no social position. And Imogen was the King's daughter. The King forced Posthumus out of the kingdom.
Iachimo bets Posthumus that he can tempt Imogen to commit adultery. Iachimo goes into her bed and steals her bracelet pretending he had sex with her, then tells Posthumus. Posthumus makes his servant Pisanio murder Imogen. Pisanio warns her and helps her fake her death, and then disguises her as a boy. He takes her to the West Coast of Britain. She becomes friends with Polydore and Cadwell who are really her brothers.
The backstory to the play is that 20 years ago, two British noblemen swore falsely, saying that Belarius had made a plan with the Romans, which led King Cymbeline to banishing him. Belarius kidnapped the king's young sons in retaliation-- to prevent eventual heirs. The sons were raised by the nurse Euriphile in the West Coast of Britan.
At the end, each character adds a piece of information; however the doctor's story is the most damning. Cornelius, the court doctor, tells that the Queen is dead but she confessed on her deathbed that she never loved the King; and she tried to have her daughter poisoned so Cloten, her son, would have the throne.
The play, Cymbeline ends with a speech to the gods, showing there is now peace and friendship between Britain and Rome.