Darth Plagueis is a minor character in the Star Wars series. He is mentioned but unseen in Revenge of the Sith, by which point in the chronology he has already died.
Darth Plagueis is a Star Wars novel by James Luceno that fleshes out the character's backstory and explains how he came to meet Palpatine and train him as his apprentice. It was published in 2012.
In the films
Darth Plagueis is not mentioned until Episode III, when Chancellor Palpatine tells Anakin Skywalker about him as part of his attempt to turn him to the Dark Side:
"Darth Plagueis was a Dark Lord of the Sith so powerful and so wise, he could use the Force to influence the midichlorians to create life. He had such a knowledge of the dark side, he could even keep the ones he cared about from dying ... unfortunately, he taught his apprentice everything he knew, and then one night, his apprentice killed him in his sleep."
Palpatine used Plagueis's story to appeal to Anakin, who feared that Padmé would not survive the birth of their child. Palpatine later strongly implies that Plagueis had been his master when he tells Anakin that his master taught him the secrets of life and death.
The implication in the film is that Plagueis and/or Palpatine set Anakin's virgin conception, first referenced in The Phantom Menace, in motion.
In the novel (spoilery, but not surprising if you've seen the movies)
Luceno's novel traces Plagueis (born Hego Damask)'s life from his apprenticeship under Darth Tenebrous to his death at the hands of his own apprentice, as anyone who's seen Revenge of the Sith knows. The novel confirms that Palpatine (who took the Sith name Sidious) was Plagueis's apprentice and was responsible for his demise.
Plagueis has an interest in studying midichlorians; when he discovers that Tenebrous had a would-be apprentice waiting in the wings to test Plagueis's mettle and take his place if he failed, Plagueis performs a series of experiments on the would-be apprentice, using his midichlorians to bring him back from the brink of death again and again. Sidious eventually witnesses and helps with his experiments. At one point, they are described as causing a noticeable shift in the Force. Meanwhile, Palpatine secretly nurtures his own apprentice (in violation of the "rule of two" that states that only two Sith — a master and an apprentice — can exist at any one time), Darth Maul.
Throughout the novel, Plagueis assists Palpatine as he makes strides in his political career. Their plan is for Palpatine to be elected Supreme Chancellor of the Republic, at which point he will name Damask his co-chancellor. This would set the stage for their planned purge of the Jedi and conquest of the galaxy. The novel leads up to and includes some events of The Phantom Menace, including Padmé as Queen Amidala of Naboo, the blockade by the Trade Federation, the war with the battle droid army, the discovery of Anakin and his Force strength on Tattooine and the lightsabre duel between Qui-Gon Jinn, Obi-Wan Kenobi and Darth Maul that results in Jinn and Maul's deaths.
The suggestion is that Plagueis lived until the end of The Phantom Menace, though of the characters in that film he only interacted with Palpatine during that timeframe. (He is said to have met Qui-Gon Jinn years earlier.) He nearly meets Anakin during one scene, but the boy is not in his room.
On the eve of his election, Palpatine kills Plagueis, telling him that this was always his plan and that he will rule the galaxy alone.
The novel was a decent read for my subway commute, though I found its end a bit abrupt. I realize that it's retconning, but I appreciated Luceno's attempts to clarify aspects of the prequel films that didn't make sense from the movies alone (the heretofore unmentioned Jedi Master Sifo Dyas placed the order for the clone army, as per Attack of the Clones? The Naboo monarchy is elected? Where did Darth Maul come from, anyway?). Probably for completists only.
Luceno, James. Darth Plagueis. Del Ray, 2012.