For the non-canon Palpatine see Palpatine Parodies
Palpatine or Emperor Palpatine is the very top of the villains in Star Wars, as Sauron was in Lord of the Rings. Palpatine is portrayed by Ian McDiarmid in the Episodes I,II,III,V (special edition), and VI.
In Star Wars Episode I: The Phantom Menace, set 32 years before Star Wars Episode IV: A New Hope, Palpatine is introduced as the senior Galactic Senator from the planet Naboo. The Trade Federation blockades and invades Naboo under the influence and advice of Palpatine's alter ego Darth Sidious. Queen Padmé Amidala flees to the galactic capital planet of Coruscant to receive counsel from the senator. After a plea for help from the senate results in bureaucratic delays, Palpatine persuades her to make a motion to have Supreme Chancellor Finis Valorum removed from office. Palpatine, as Sidious, sends his Sith apprentice Darth Maul to Naboo to oversee the invasion and find the queen. The invasion, however, is thwarted by Jedi Qui-Gon Jinn and Obi-Wan Kenobi; in the ensuing lightsaber duel, both Maul and Qui-Gon are killed. Palpatine returns to Naboo, having been elected the new Chancellor. He tells nine-year-old Anakin Skywalker that he will be "watching his career with great interest".
In Star Wars Episode II: Attack Of The Clones, set 10 years later, the galaxy is on the verge of civil war, as a growing Separatist movement of planets seeking to secede from the Republic to form the Confederacy of Independent Systems. They are led by Count Dooku, a former Jedi and Darth Sidious' new apprentice. After Kenobi discovers that the Separatists are building a secret battle droid army, Palpatine uses the situation to have himself granted emergency powers. Palpatine feigns reluctance to accept this authority, promising to return it to the Senate once the crisis has ended. His first act is to create a Grand Army of the Republic of clones to counter the Separatist threat. The clones had recently been discovered by Kenobi as having been secretly ordered by deceased Jedi Master Sifo-Dyas many years earlier. Palpatine is also influential in having Anakin guard Padmé, which leads to their marriage at the end of the film.
In Star Wars Episode III: Revenge of the Sith, set three years later, Palpatine is captured by Separatist leader General Grievous. Palpatine is rescued by Kenobi and Anakin Skywalker, but not before the Jedi confront Count Dooku; Skywalker decapitates the Sith apprentice in cold blood during the lightsaber duel, at Palpatine's urging. By this point, Palpatine has used the crisis to stay in power long after his term expired, and has acquired virtually dictatorial authority in the Senate. The Jedi Council, as well as a number of Senators, are troubled by Palpatine's power and fears he will not relinquish it when the Clone Wars end. Palpatine raises the Jedi's suspicions further when he has the Senate grant him direct control over the Jedi Council, and appoints Anakin as his personal representative, effectively granting him a vote in Jedi affairs. He begins to tempt Anakin towards the dark side, and tries to turn him against the Jedi by suggesting that he deserves the rank of Jedi Master, which the Council refuses him. The Council orders Anakin to spy on Palpatine, but he instead reveals the Jedi's plan to him. Palpatine tells Anakin the story of Darth Plagueis, a powerful Sith Lord who was able to manipulate life and death, but was killed by his apprentice (hinted to be Sidious himself). Palpatine reveals his secret identity to Anakin and tempts him with promises of power over life and death. Palpatine knows that Anakin has been having visions of Padmé, who is now pregnant with Anakin's child, dying in childbirth and offers to teach him the secrets of Darth Plagueis to save her life. Confused, Anakin informs Jedi Master Mace Windu that Palpatine is a Sith Lord. Windu and a group of fellow Jedi Masters go to arrest Palpatine, but the Chancellor surprises them with a lightsaber and quickly dispatches all but Windu. In the ensuing duel, Windu gains the upper hand, deforming Palpatine's face by deflecting his own Force lightning back at him, and is poised to execute the Sith lord — when Anakin appears and intercedes in Palpatine's behalf, cutting off Windu's hand. Palpatine then kills Windu with another blast of lightning, and accepts Anakin as his new apprentice, Darth Vader.
Palpatine then sets the destruction of the Jedi in motion: he sends Vader to destroy the Jedi Temple and instructs all of the clone troopers to kill their Jedi generals. He then sends Vader to Mustafar to wipe out the Separatist leaders. He announces to the Senate that the Jedi were planning to overthrow the Republic, and that the Republic will be reorganized into the Galactic Empire, with himself as Emperor for life. Jedi Master Yoda returns to Coruscant and confronts Palpatine in his Senate office. A lightsaber duel erupts between them which ends in stalemate, forcing Yoda to retreat into exile. Sensing his apprentice is in trouble, Palpatine travels to Mustafar, where he finds Vader maimed and burned almost to the point of death following a duel with Kenobi. Palpatine returns to Coruscant with Vader and provides him with a black armor suit and artificial limbs. When Vader regains consciousness, Palpatine tells him that he (Vader) killed his own wife in anger, breaking what remains of his spirit. Palpatine is last seen watching the first Death Star under construction with Vader at his side.
In A New Hope The Emperor is mentioned briefly.
As the Emperor, Palpatine made his first appearance in The Empire Strikes Back as the leader of the Galactic Empire, and Sith master of Darth Vader. The Emperor contacts Vader by holographic communication to tell him of a "great disturbance in the Force," and warn him that Luke Skywalker is becoming a threat (he is identified only as the "offspring of Anakin Skywalker" in the modified scene in the Special Edition). Vader convinces the Emperor that Skywalker would be an asset if he could be turned to the dark side.
Star Wars Episode VI: Return Of The Jedi
In Return of the Jedi, the final episode of the original trilogy (and chronologically the entire Star Wars film saga), the Emperor arrives on the second Death Star to oversee the last stages of its construction. When a Rebel strike team that includes Skywalker lands on nearby Endor, Vader senses Skywalker's presence. Skywalker believes he can save Vader, his father, from the dark side, and the Emperor believes that this hope will lead to his downfall. Luke surrenders to Imperial forces on Endor and is delivered to the Emperor. The Emperor, who plans to replace Vader with Luke as his apprentice, tempts the young Jedi to the dark side by appealing to his fear for his friends. This leads to a lightsaber duel in which Skywalker defeats Vader. Skywalker, however, refuses to turn to the dark side, and the Emperor attacks him with Force lightning. At the last moment, Vader turns on his master and throws him into the Death Star's reactor shaft, killing him.