So I've been reading Star Wars: Labyrinth of Evil. Great book so far...obviously. I just passed this part in the book talking about Sidious, and it says, "Of his youth, Sidious had offered little these past thirteen years; of his Master, Darth Plagueis, even less." So we know for a fact that Plagueis not only did exist, but was in fact Sidious's Master.

Now onto the interesting stuff.

At the opera house, Palp told Anakin the story of Plaguies. We all know the story, and why Palp told Ani.

When Anakin turned to the dark side, Sidious said that only one knew how to cheat death...but at the opera house, he said that Plaguies taught his apprentice everything, which meant two people knew how to cheat death.

So here's my theory: Plaguies never knew how to manipulate the midi-chlorians or the force to create life or prevent death. The story was just a ploy that Palp/Sidious used to lure Anakin to the dark side. Obviously, it worked.