One of William Shakespeare’s most original and intricate plays, Cymbeline, King of Britain is a tragicomedy about a semi-legendary king at the time of the Roman occupation, written in about 1610. Its elaborate plots and obscure subject have led to it being relatively unpopular, and it has never been revived for long.
As one of Shakespeare’s accounts of English history, it could have formed a good theme for a new British School of History Painting in the late eighteenth century, but never seems to have been an attractive basis for visual art.
Cymbeline, vassal King of Britain, had three children. His two sons were abducted as infants by the traitor Belarius. His daughter Imogen (or Innogen) has secretly married a member of his court, Leonatus Posthumus. The couple exchange jewellery as tokens of their faithfulness, so he now wears her diamond ring, and she the bracelet he gave her, when they are interrupted by her furious father, who banishes Posthumus to Rome.

Henry Justice Ford shows the couple together in his undated watercolour of Posthumus and Imogen, with the forward reference to his departure by sea to Rome.

John Faed’s Posthumus and Imogen from about 1865 shows him placing the bracelet on her arm.
When Posthumus reaches Rome, he bets his host Giacomo the diamond ring and ten thousand ducats that the Italian can’t seduce his bride Imogen.
Back in Britain, the Queen procures poison from Doctor Cornelius, but as he doesn’t trust her, he actually provides a potion to induce deep unconsciousness instead of death. She hands it to Posthumus’s servant Pisanio, telling him it’s a powerful medicine.
When Giacomo arrives in Cymbeline’s court he almost manages to convince Imogen that Posthumus has been unfaithful to her in Rome, but gives the game away when he tries to supplant her husband. He succeeds in persuading her to keep his large trunk in her private chamber. When she has fallen asleep in bed that night, Giacomo emerges from his trunk to make notes about the interior of the room, steal her bracelet from her wrist, and notice a distinctive birthmark on Imogen’s left breast, before getting back into his trunk.

Wilhelm Ferdinand Souchon’s painting of Imogen asleep in bed, from 1872, makes the viewer voyeur and assume the role of Giacomo, although the birthmark isn’t obvious. She is also thoroughly modern in appearance, with a part-read book resting on a small table next to her.
In the morning, Imogen’s former suitor, the inept clod Cloten, has her serenaded by musicians with the song Hark, hark, the lark, but she just insults him in return.
Giacomo travels back to Rome to show Posthumus his wife’s bracelet and describe her birthmark, so convincing Posthumus that he has won his bet. He receives the diamond ring as his prize, and Posthumus rages against women.
Cymbeline is told by the Roman ambassador to pay his annual tribute to Augustus Caesar, but the Queen persuades him to refuse, causing Rome to declare war on Cymbeline’s kingdom.
Pisanio, Posthumus’s servant, receives two letters from Posthumus. The first instructs him to kill Imogen for her infidelity, the second, addressed to Imogen, tells her that he is waiting for her at Milford Haven in Wales, providing Pisanio with the opportunity for her murder.
Meanwhile, outside the cave in Wales where they live, the banished Belarius warns his two adopted sons, whom he had abducted from Cymbeline as infants, about life at court.
As Imogen and Pisanio approach Milford Haven, the servant finds he is unable to follow his master’s orders, so shows Imogen his letter from Posthumus. She is outraged with her husband, renounces him and tells Pisanio to kill her forthwith.

This engraving of John Hoppner’s painting of Pisanio and Imogen was made in 1801 for Boydell’s Shakespeare Gallery prints. Imogen is handing Pisanio a sword with which to kill her.
Pisanio intends to send Posthumus bloodstained clothing as evidence of Imogen’s death, advises her to disguise herself as a page and travel to Rome with the Roman ambassador, and gives her the strong medicine that the Queen gave him.
The King and Queen part from the Roman ambassador before learning of Imogen’s flight. Cloten, her suitor, reads Posthumus’s letter inviting her to Milford Haven. He decides to dress as Posthumus, travel to Milford Haven, and there kill Posthumus and rape Imogen.

Another engraving, this time made from Richard Westall’s painting of Imogen in 1803, was included in Boydell’s collection.
Disguised as a page, Imogen is hungry and discovers the cave of Belarius, where she is welcomed by the three men. When they go hunting, she feels unwell, so remains at the cave alone and takes the medicine provided by the Queen. Belarius and the two sons come across Cloten, who fights with them and is quickly beheaded. When they return to their cave, they find the apparently dead body of Imogen, and lay her corpse out while singing a dirge. They put Cloten’s headless body next to her before departing.

Edward Penny’s Imogen Discovered in the Cave from about 1770 is slightly incongruous, showing Belarius and the two sons outside the cave, and Imogen standing inside.

George Dawe’s Imogen Found in the Cave of Belarius, from about 1809, fits Shakespeare’s account better. This painting won the prize for Historical and Poetical Subjects at the British Institution exhibition of 1809.

James Smetham’s far later Imogen and the Shepherds, from 1874, is more enigmatic still, with two shepherds grieving over the apparently lifeless body of Imogen.
Imogen recovers consciousness only to see what appears to be the headless corpse of Posthumus beside her, and presumes that Pisanio and Cloten conspired against her. When the Roman ambassador passes by, on his way to Milford Haven, he assumes that she is a page weeping over the death of her master, and takes her into service as Fidele.
Cloten’s absence worries the Queen; Pisanio has heard nothing from either Posthumus or Imogen, and the King learns that the Roman army has landed. Belarius and the two sons decide to join the British army, as does Posthumus, who has arrived in Britain with the Romans and still carries the bloody fabric as evidence of Imogen’s death.
When the Britons fight the Roman army, Posthumus defeats Giacomo, but spares his life. Cymbeline is captured by the Romans, then rescued by Belarius, the two sons and Posthumus. The Roman army is forced to retreat. Posthumus decides to dress as a Roman in the hope of being killed, but is taken prisoner instead. Begging Imogen’s forgiveness, he falls asleep and dreams of his family calling on Jupiter to save him. The god promises that he hasn’t abandoned him, and leaves him a tablet of riddles which Posthumus is unable to interpret. Posthumus is then summoned to the King.
Cymbeline is rewarding Belarius and the two sons for their accomplishment in battle, but can’t find their comrade Posthumus. He is then told of the Queen’s death, and of her dying confession that she planned to murder both Imogen and Cymbeline so that Cloten could usurp the throne.
Posthumus, Giacomo and Imogen (still disguised as Fidele) are brought before the King, each expecting to be killed. The Roman ambassador obtains clemency for Fidele, but she has recognised Giacomo by the diamond ring he’s wearing. She demands to know how he obtained it, and elicits his confession of cheating in his wager with Posthumus. Posthumus is enraged by his admission, and won’t be interrupted by Imogen.
Pisanio recognises Imogen, who accuses him of giving her poison. The doctor explains that he had provided it to the Queen, who had confessed that it was she who passed it on to Pisanio. Posthumus, Imogen and the King embrace before Pisanio and one of the sons explain Cloten’s fate. For that, Cymbeline unwittingly sentences his own son to death, until Belarius reveals their true identities.
Cymbeline pardons them all, and a soothsayer explains the meaning of the riddles on the tablet given to Posthumus as prediction of this reunion of Cymbeline’s family. Finally, the King sues for peace by resuming payment of his tributes to Rome.
References
Wikipedia on Shakespeare’s play.
Full text at Project Gutenberg.
Michael Dobson and Stanley Wells (eds) (2015) The Oxford Companion to Shakespeare, 2nd edn, Oxford UP. ISBN 978 0 19 870873 5.