Will anyone ever grow tired of seeing Dame Helen Mirren channel the life of a monarch? Well, seeing as the Academy Award winner just stepped into the role of Catherine the Great for HBO’s new four-part historical drama about the Russian empress, I'm going to go with "no." At the center of the show, which premieres October 21, is the stormy affair between Catherine and Russian military officer Grigory Potemkin (Jason Clarke) and how it affected her reign. Even the most major history buff might not be familiar with this steamy fling. Or, maybe you just want to double check the facts before you dive to far into the series. Either way, here's what you need to know about the real relationship between Catherine and Grigory.