Ideally, the rule is to assume that every story may be someone's first -- or at least that returning viewers/readers might need to be reminded of things they've forgotten. So every installment of a series should remind people of whatever they need to know to follow the story. The goal is not to create obstacles to entry or punish people for not knowing the "right" things. You want your work to be accessible to everyone, not just the experts.
After all, there's no reason audiences can't follow a story that references events they didn't experience. People in 1977 could follow Star Wars just fine without prior knowledge of the Jedi or the Clone Wars. People could follow "The Cage" without having seen the battle on Rigel VII beforehand or having known the dead yeoman Captain Pike was mourning. It's not that hard to fill audiences in on unseen past events with just a few brief references. So by the same token, it's not hard to make similarly brief references to the events of previous installments, either to remind returning audiences without photographic memories or to bring new audience members up to speed. Certainly prior familiarity can make details and nuances clearer, add deeper context, etc., the way the prequel trilogy added context to Star Wars or the way "Among the Lotus Eaters" revealed more about Rigel VII and Pike's yeoman. But it should never be required for basic understanding of who the characters are and what's happening in the story. It's the storyteller's job to include any essential information in the current story itself.
So there's simply no validity to the notion that an audience ever "has to" be familiar with previous installments to follow the current one. Or rather, if an installment isn't comprehensible without that prior knowledge, that's a failure of basic story construction.
