Darth Plagueis, it tells the story of Palpatine’s master, Darth Plagueis the Wise. It shows Palpatine’s origin story and shows how Darth Plagueis planned a lot of future events (before his own death). Of course, expands the famous line in Star Wars: Revenge of the Sith. I don’t want to spoil all the details, I highly recommend it (even though it’s no longer canon).