Darth Plagueis was a Sith Lord who, according to legend, had discovered a way to manipulate midi-chlorians to prevent death and create life. However, his apprentice killed him in his sleep. Although it is never officially stated in the cinematic release of Revenge of the Sith that Plagueis was Darth Sidious' master, the official Star Wars Databank characterizes him as such. This information is included in the film's novelization, in which Sidious says, "Darth Plagueis was my master... before I killed him." It is also mentioned in the Star Wars novels Labyrinth of Evil and Dark Lord: The Rise of Darth Vader. "The Tragedy of Darth Plagueis the Wise", as related by Palpatine, is the only example of in universe literature contained within the six Star Wars films.

In Revenge of the Sith, Sidious, in his civilian appearance as Supreme Chancellor Palpatine, recounts Plagueis' story to Jedi Knight Anakin Skywalker in order to lure him further toward the dark side of the Force. Palpatine's explanation of Plagueis's power to Anakin was as follows:

“ Palpatine: You ever hear the Tragedy of Darth Plagueis the Wise?
Anakin Skywalker: No.
Palpatine: I thought not. It's not a story the Jedi would tell you. It's a Sith legend. Darth Plagueis was a Dark Lord of the Sith, so powerful and so wise he could use the Force to influence the midi-chlorians to create life(looks at Anakin)... He had such a knowledge of the dark side he could even keep the ones he cared about from dying.
Anakin Skywalker: He could actually save people from death?
Palpatine: The Dark Side of the Force is a pathway to many abilities some consider to be... unnatural.
Anakin Skywalker: What happened to him?
Palpatine: He became so powerful the only thing he was afraid of was...losing his power, which eventually, of course, he did. Unfortunately, he taught his apprentice everything he knew, then his apprentice killed him in his sleep. Ironic; he could save others from death, but not himself.
Anakin Skywalker: Is it possible to learn this power?
Palpatine: ...Not from a Jedi. "



In the Expanded Universe novel Dark Lord: The Rise of Darth Vader, a "sequel" to Revenge of the Sith, Palpatine muses on Plagueis' philosophy of the Force (he also explicitly identifies the late Sith Lord as his former master). Plagueis' philosophy is recounted as follows:

“ Tell me what you regard as your greatest strength, so I will know best what to undermine you. Tell me of your greatest fear, so I will know what I must force you to face. Tell me what you cherish most, so I will know what to take from you. And tell me what you crave, so that I might deny you."