There are two main lines: the story of the lovers of Postum and Imogen and the story of lost and found royal sons. All this, like a silver frame, is framed by sufferings, oaths of loyalty, amazing coincidences and other tasty Shakespearean curlicues. Postum was simply naive and blind, like most of the heroes of Shakespeare. And Imogen, in my opinion one of the most intelligent and clever heroines of him.
