The hour will make you wish you had done some rewatching beforehand, so allow us (with some help from Rothenberg) to give you some suggestions.
While season three, episode seven, “Thirteen,” is a tough one for many fans due to the death of Lexa, it does contain important information about the origins of the apocalypse and how that relates to that pesky Flame.
Season four, episode three, “The Four Horsemen” introduces us to Bill Cadogan, leader of Second Dawn, and season four, episode eight includes the discovery of the real Second Dawn bunker, located under Polis, which was mysteriously empty.
Season five, episode 10, called “The Warriors Will,” shows what eventually happened to Becca through a vision, revealing that he was burned at the stake by Cadogan and Second Dawn.
All of this will be relevant in some way to the episode and to the potential future series.