Star Wars Teases When Palpatine Killed His Master, Darth Plagueis
A Star Wars tie-in has suggested exactly when Palpatine, or Darth Sidious, killed his master, Darth Plagueis. First mentioned in Star Wars: Episode III - Revenge of the Sith, Darth Plagueis has long been one of the most mysterious figures in Star Wars. Palpatine tells Anakin the story of "Darth Plagueis the Wise" as a means to entice him over to the dark side, with a tale of how is master looked to learn the secrets of cheating death. While Palpatine's manipulation worked, and the would-be Emperor himself later did find a means to eternal life, Plagueis himself never quite managed the feat - because of his apprentice.
In the grand tradition of the Sith's Rule of Two, Palpatine killed his master to ascend further as a Sith Lord, which included taking on his own apprentice in the form of Darth Maul. Star Wars canon has since left Plagueis' history a decidedly murky subject. There was a book written about him, James Luceno's 2012 novel Darth Plagueis, but it was rendered non-canon by Disney's purchase of Lucasfilm, and with it much of Plagueis' backstory was gone too.
Star Wars is starting to add more to Darth Plagueis story bit by bit though, including in the book Secrets of the Sith. Written from the in-universe perspective of Emperor Palpatine, it sheds light on all things dark side, delving into his own past and many different aspects of the Sith, their powers, origins, and various other elements. This includes, unsurprisingly, his relationship with and murder of Darth Plagueis. In the book, Palpatine reveals some of his thinking when he killed his master, and hints at it happening before Star Wars: Episode I - The Phantom Menace, though it could still have been the same year, but perhaps even earlier:
"After centuries in the shadows, the Sith were poised to emerge once more. As a dark apprentice, I sensed that the Galactic Republic was in disarray. Its delegates held no interest in the common good. They would be easy targets for my influence. Serving as a senator, I manipulated their greed in my favor. With each new act of legislation, I rose through their ranks. Total control of the galaxy's governing body drew ever closer. But ascension in the Senate would not be enough. My master, Darth Plagueis, had served his purpose. I struck him down, claiming his place as the one true master of the Sith."
Palpatine's ascension in the Senate fits with his operations in The Phantom Menace, where he ultimately rises to the position of Chancellor. His words here, though, suggest this takes before the Star Wars prequels as he's still very much rising through the ranks, working on new acts of legislation, and so it's likely before the events of The Phantom Menace. This makes some sense from a canon perspective, as it gives a much clearer indication of why there was no sign of Plagueis during The Phantom Menace, with Darth Sidious having taken on his own apprentice by that point instead. It makes for a much neater timeline too, whereby any further exploration of Darth Plagueis has a clear end point, allowing future stories to go up to, but not overlap with or contradict, the prequels.
This reveal is a change from Star Wars Legends, as Luceno's novel established a different timeline for Plagueis death. In the book, it's only after Palpatine has made sure he will be elected Chancellor that he decides to kill Darth Plagueis, believing he would only get in the way of his lofty ambitions and seeing the opportunity to grow even more powerful by slaying him. As seen in the prequels, Palpatine had grand plans for both the Senate and the Sith, and he wouldn't let anyone, not even his master, get in the way.