For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Imogen is married to Posthumus but no one is happy about it. He is banished, Imogen is heartbroken, and the Queen unleashes her fool of a son on the princess. Imogen defends herself against his advances, as well as those brought on by Iachimo from his bet with her husband. Somehow, she still ends up labeled a whore and her husband wants to kill her. Rome decides to invade Britain and it doesn't look good for Cymbeline's home country.
Act II
After disguises and fighting galore, Imogen is thought to be dead by just about everyone. Posthumus is in jail for fighting with the Romans (even though he saved the British king). Imogen thinks he is dead. Cymbeline thinks his sons are dead since they were kidnapped 20 years ago.
Act III
Posthumus is rescued from prison and congratulated for saving the king's life. Imogen reveals herself and everyone is overjoyed she is alive. The lovers are reunited after Iachimo tells them of his nasty plot against them. And miraculously, Cymbeline's missing sons are discovered. It's almost too good to be true for the characters. Almost.