In Act One, British king Cymbeline’s daughter Imogen has enraged her father by marrying Posthumus; instead, the king wanted her to marry the son of his new wife. The king has imprisoned bride and banished the groom. We learn that Imogen has two older brothers both kidnapped as toddlers, whereabouts unknown. In Rome, Posthumus speaks lovingly of Imogen’s beauty and virtue. A local, Iachimo challenges Posthumus: Iachimo will travel to Britain and will attempt to sleep with Imogen. If he succeeds, Posthumus will give up his ring (one given to him by Imogen); if Iachimo fails, then he must pay Posthumus ten thousand ducats, plus agree to a duel that Posthumus demands. Back in Britain, the Queen attempts to hire Pisanio away from Posthumus, even offering what she says a curative potion, only she thinks it’s a poison, but it’s actually one that will resemble death. As Imogen bemoans her state, Iachimo arrives, intimating Posthumus as been less than faithful in Rome, and suggests that she sexually revenge herself on Posthumus. She immediately calls Iachimo out on his deceit; he apologizes, and she agrees to storing Iachimo’s trunk in her room overnight.
In Act Two, when Imogen goes to bed, that chest opens and he emerges. He creeps about her room, talking of touching her, kissing her, writing down everything he sees, in hopes of using these details to win his bet. He takes the bracelet Posthumus gave Imogen, and takes note of a mole on her left breast. He then pops back into the trunk as daylight comes. Cloten and his lords arrive beneath Imogen’s window for a serenade. It really doesn’t work, and she tells him that she cares for Posthumus’ worst suit more than for Cloten. In Rome, Iachimo convinces Posthumus of Imogen’s infidelity.
Act Three begins with the Britons’ refusal of Rome’s demand of tribute, presaging war. Pisanio receives a letter from Posthumus demanding that the servant murder Imogen for her infidelity; he also sent one to Imogen, directing her to Wales, where he claims to have sneaked back into the country (and where Pisanio can kill her). In the woods of Wales, the king’s kidnapped two sons and their kidnapper are living as a family. Imogen and Pisanio arrive in the woods, where Imogen asks repeatedly for Posthumus, and a tormented Pisanio gives her his letter from Posthumus. She is incredulous, turning from shock to anger. Pisanio suggests that she dress as a man and stay in the woods, and present herself to Lucius (whose Roman army is heading to Wales), join his service, and thus gain access to transportation away. In case she gets injured, Pisanio gives her the drug the queen gave him. Pisanio returns to the castle and is questioned by the Queen’s son. Cloten reads Imogen’s letter from Posthumus, then orders Pisanio to bring Posthumus’ clothes: he’s going to go to Milford Haven, dressed as Posthumus, rape Imogen, then kill Posthumus in her presence, then drag her back to the castle. Back in the woods of Wales, Imogen and the three woodsmen meet. They ask her to join them, treating her like a brother.
Act Four of Cymbeline opens with Cloten arriving in the Welsh woods, wearing Posthumus’ clothes, and noting in soliloquy how well they fit him. Then he reiterates his plan to kill Posthumus and rape Imogen. The second scene opens with the unknowingly reunited “brothers” of the king (Imogen is pretending to be a boy, Fidele). Imogen is not feeling well, so the brothers tell her to stay in the cave. She takes some of “the potion” and enters the cave, while the three remaining men continue praising the youth in her/his absence. Into this scene enters Cloten. Belarius (faux father) recognizes him, and instantly worries that Cloten’s brought others; he and the younger brother Arviragus go looking, while Cloten tries to intimidate the other brother, Guiderius. Cloten insults the heir, the heir doesn’t take it kindly, and the two exit the stage fighting. Meanwhile, “father” and brother return from finding no one, to find their place abandoned, too. And Guiderius returns with Cloten’s head. Belarius freaks out. Guiderius leaves to dispose of the head by throwing in the creek. The little brother goes to wake Fidele/Imogen for dinner, only to return with “dead Fidele” in his arms. They decide to bury Fidele and Cloten; Belarius retrieves Cloten’s body and lays it next to Imogen, and the three of them leave to dig the graves. Imogen wakes up, a little disoriented. She finds the dead headless body next to her, and recognizes the clothes. When she ponders what has happened, she jumps to the conclusion that Pisanio and Cloten joined forces, Pisanio using the knock-out drug on Imogen, while Cloten found, killed, and decapitated Posthumus. Enter Lucius and his Roman army and they find Imogen over the body of Cloten. “Fidele” says that this body is “his” master’s; Lucius, impressed by the boy’s loyalty, takes the boy into their fold for protection, and orders his soldiers to bury the body. The third scene of the act has Cymbeline musing on the Queen suffering an illness over the disappearance of her son. The fourth and final scene of the fourth act takes us back to Wales, where war is approaching. The two younger men convince Belarius to join with the Britons and fight for their nation.