Darth Sidious is a fictional character in the Star Wars universe.
A Sith Lord, Darth Sidious was the prime practitioner of "The Dark Side of the Force". In Star Wars Episode I: The Phantom Menace, Sidious sent out his apprentice Darth Maul to kill Queen Padme Amidala and the Jedi knights Qui-Gon Jinn and Obi-Wan Kenobi.
In Star Wars Episode II: Attack of the Clones, Sidious again plotted against the Jedi, this time sending his new apprentice Count Dooku, a.k.a Darth Tyranus to do his work.
Later, supposedly in Star Wars Episode III: Revenge of the Sith, it is generally assumed by fans that Sidious will be revealed to be actually Chancellor Palpatine, and take on Anakin Skywalker as his new apprentice under the name Darth Vader. If so, then between Episode III and Episode IV, Darth Sidious will become the Emperor who is seen in Episodes V and VI.
There are clear clues to this fact that have been shown earlier, for example in the closing scenes of The Phantom Menace, Palpatine is shown in profile, matching Sidious seen in the same position but with his face hidden by the hood of his cloak. Palpatine and Sidious are also, not coincidentally, both introduced in The Phantom Menace as holograms and Sidious makes numerous references to his direct ties to the senate but never actually tells his underlings what they are. Look closely when Sidious and Maul speak on Coruscant: the railing balcony matches the one seen from Senator Palpatine's apartment later in the film. Both Darth Sidious and Palpatine are also played by the same actor, Ian McDiarmid. Many reference materials made after 2002 blatantly refer to The Emperor as Darth Sidious (such as the Unleashed toy and Inside The Worlds of The Star Wars Trilogy), but still no in-story connection has been explicitly made.
In the original movie trilogy, the name Palpatine is never spoken and he is referred to only as "The Emperor". The name Palpatine originated in the novelizations, in which the name is given as that of the former senator who rose to power.
"Sidious" is two letters away from being "insidious ," a word which describes Darth Sidious very well.
In Revenge of the Sith, the true extent of Sidious's plan is revealed. His reasons for playing both sides of the Clone Wars against each other - with himself as Palpatine controlling the Republic legitimately and his apprentice Tyranus as the head of the CIS - are revealed to be an elaborate scheme designed for the primary purpose of eliminating the Jedi. Also part of this plan is the establishment of a New Order- the Galactic Empire- led by Palpatine and his apprentice.
The Chancellor uses his friendship and trust with Anakin Skywalker to corrupt him to the dark side. He convinces Anakin that the Jedi and the Senate can not be trusted, but the primary motivating factor for Anakin is Palpatine's suggestions that he could save Padme, Anakin's beloved - since Anakin foresees her death in her imminent childbirth. Anakin - now really more properly known as Darth Vader - duels with his former master, Kenobi, over the lava flows of the planet Mustafar, culminating in Anakin's fall into the molten lava. Palpatine rescues Vader from the lava after much of his body is incinerated and saves him by having him placed in a life-sustaining armored suit- the suit that Vader is seen wearing throughout the classic trilogy.
The Star Wars backstory
Sidious is the Sith Lord responsible for accomplishing the ultimate goals of that dark and clandestine order. Namely, the complete destruction of the Jedi Order and the restoration to galactic power of the Sith. These have been Sidious' motivations, and these he has sought to implement with every stroke.
It is unknown when he became Dark Lord of the Sith, but the name of his master is known: Darth Plagueis. It is known that as early as a decade prior to the events of Episode I, Sidious had begun training his apprentice Darth Maul. Sidious, not content to just convert a force user to the Darkside, instead chose to mold and carefully sculpt his right hand from scratch. Maul was kidnapped as an infant; trained personally by Sidious in all forms of combat and use of the Force; and especially, was taught aggression, anger, and hatred. Seemingly much of Sidious' efforts went into perfecting Maul into the ultimate weapon. But in reality, Maul was just another pawn in Sidious' grand strategy.
Sidious, possessed with superior intellect, had a cunning understanding of the human mind and great use of the Force in foreseeing the future, so he could manipulate events as though he were a dejarik grandmaster moving pieces on a board. Darth Sidious had a broad expertise in areas such as psychology, bureaucracy, and philosophy. As Palpatine, he had gained a reputation for being a good and modest man, and had managed to get himself elected as Senator from the relatively minor planet of Naboo. Leading up to the events of Episode I, Palpatine had judiciously used his intellect to become a well-respected politician in the Senate, and had even gained the friendship of the current Chancellor, Finis Valorum.
In the background however, Sidious unfurled his plan. Part of it was to train Maul; another part of it was to then have Maul assassinate key figures, thus gaining influence for Sidious, especially in illicit circles.
This eventually led to a secret alliance with the Trade Federation. The Trade Federation was a consortium which controlled shipping throughout the Galaxy that had become powerful enough to have full representation in the Senate as though they were a member world. The Senate had voted to levy taxes on trade in order to raise funds for the government (this at the manipulation of Palpatine). Sidious, appealing to the Trade Federation's greed, persuaded them to blockade Naboo in protest of the levy. He assured them that the action would bring them greater control and profits, as the Senate would be impotent to act.
Meanwhile in his role as Palpatine, he convinced Amidala, queen of the Naboo, to endure the blockade. Advising her that the Trade Federation could not force the issue, and that the Senate would not let the dispute continue for long. This leads to the events opening Episode I.
Jedi mediators had been dispatched to settle the conflict (at the request of the Chancellor, and the persuasion of his friend Palpatine). When it seemed as though the Trade Federation would be forced to settle or else face the Jedi, Sidious ordered them to kill the Jedi instead and invade Naboo, assuring them that it would be over before the Senate could respond. Sidious' real goals were to weaken the power of the Trade Federation (one of the key galactic forces at the time), and to gain sympathy among the Senate due the invasion.
As Sidious had intended, the Jedi rescued Amidala and brought her to the Senate. There, she testified of what the Federation had done and pleaded for help. Amidala's reputation and testimony was enough to convince the Senate to take action. Since Chancellor Valorum was unwilling to do anything at that time, however, so Amidala asked for a vote of no confidence. The majority of the rest of the Senate agreed, and Valorum was forced to step down from his position.
The sympathy for Naboo concerning the ordeal was enough to earn the already well-respected Palpatine a nomination to succeed Valorum as Chancellor. His secret alliances further assured that he would be elected. Subsequently, Amidala left returned to Naboo with the Jedi, where the Trade Federation was ultimately defeated militarily, and consequently suffered a significant financial loss.
Sidious's secondary plan was not as obvious as the first (the Sith Lord had a penchant for the most complicated plans, so his grand scheme had many layers), hence it was even more brilliant and devious. Sidious knew he would need a force user of extraordinary power if he was ever to defeat the Jedi Order. The Sith Lord took the necessary steps to ensure that such a force-user was at his command.
The true extent of Sidious's influence on Anakin's life is unknown, but it is probable that Sidious was aware of Anakin from conception. Tatooine happened to be out of range of the Jedi order, and thus Anakin would be unrecruited and unknown to the Jedi. Tatooine also happened to be a harsh place, a place where Anakin would grow up an alien, a slave, and without a father. A perfect place for the seeds of the dark side to grow inside him.
Anakin was destined to become the greatest force-user in the Galaxy, and clearly who Sidious would choose as his apprentice. It is equally clear that Sidious would not be able to personally train Anakin. Sidious had many demands and plans yet to implement, especially now that he was Chancellor. So Sidious arranged other means for his future apprentice to be trained, namely the Jedi Order.
The entire Jedi Council refused to let the boy be trained. They did not know why exactly, but they did know a dark cloud surrounded the boy. Yoda said it clearest when he explained the path to the Dark Side, which begins with fear, and how he saw much fear in Anakin.
Sidious unleashed Maul, and had him track down the Jedi who were protecting Amidala. In the climactic battle of Episode I, Maul proved his prowess and defeated Jedi Master Qui-Gon Jinn. Obi-Wan however, seemed to have a better connection to the force, and was able to survive despite his foe's superior skill. This cost Sidious decades of work, but in the end he considered it worthwhile. Qui-Gon's dying wish was that the boy be trained; a wish which even the Jedi Council was unwilling to deny their fallen hero. And so an ill-prepared Obi-Wan became Anakin's teacher despite the council's concerns. And so Sidious achieved his intermediate objectives. Gifted with an uncanny amount of patience, it would be an entire decade before the next step in his sinister plan would come to fruition.
In Attack of the Clones, Sidious is seen again at the end of the movie, after Count Dooku escapes from the Battle of Geonosis and arrives via solar sail to Coruscant. Dooku lands in The Works, where Sidious is hiding out at the moment, and meets with him where they discuss their plans for the future, to be seen in Revenge of the Sith. But Dooku secretly reveals information about Darth Sidious to the Jedi who decided to keep a closer eye on the Galactic Senate.