In the fourth Lunar Chronicles book the crew head to Luna so Cinder can claim her birthright as Queen Selene. Kai helps sneak them up there but thing quickly spiral out of control, and they all get seperated. Luckily Princess Winter helped Cress avoid detection. Winter is the stepdaughter of Levana and suffers from Lunar Sickness because she refuses to glamour people. Levava is jealous of her because the people love Winter for her kindness, (and because even without a glamor she is stunning) and Levana eventually tries to have her killed. Meanwhile, Cinder and the others regroup at Wolf’s childhood home and start working out how to start a revolution among the people. Because the only way to beat Levana is with a full on uprising.
This is a great conclusion to the series although there are many short stories that are set in the same universe. It’s based on Snow White. A lot of people think of “Levana’s Tale” as the fourth book but really it’s a prequel and it doesn’t advance the story and you don’t need it to understand and enjoy “Winter” so it’s better read afterward.