I can’t really think of a good way to do this that DOESN’T involve contradicting the first two movies, but I’ll try my damndest.
The movie opens seven years after “Attack of the Clones.” The opening crawl explains that the Clone Wars are over. Palpatine exerts essentially dictatorial control over the galaxy.
The movie opens with a Republic star destroyer orbiting Alderaan. Jedi Knights Obi-Wan Kenobi and Anakin Skywalker, now reknowned across the galaxy for their exploits during the Clone Wars, and Senator Amidala are visiting Bail Organa, head honcho of Alderaan, who has sent them an urgent but secret summons. Upon arriving they are attacked by a team of assassins bent on slaying Amidala, but Obi-Wan and Anakin defeat them. It is shown through this action sequence that Anakin is a Jedi of seemingly unprecedented awesomeness. Organa tells them that he feels Palpatine is intent on assuming permanent tyrant status and asks the Jedi order’s help. They say they’ll take it back to the council and leave Amidala on Alderaan under Organa’s protection.
On their return, however, the starship they’re on is inexplicably rerouted towards a black hole. An exciting sequence with awesome special effects follows during which the crew, with Obi-Wan’s help in some Jedi way, managed to rev the engines up again to get the ship away from the event horizon. When it’s over, Obi-Wan discovers that Anakin is nowhere to be found.
Meanwhile, Anakin has returned to Coruscant and goes straight to Palpatine. We learn that Anakin is now loyal to Palpatine above the Jedi, and tried to destroy the Star Destroyer to prevent Organa’s message from getting through. Palpatine tells Anakin that he must slay his wife and Bail Organa. Both think that Obi-Wan is dead. Anakin reluctantly agrees.
Obi-Wan returns and informs the Council of what happened. They dither and do nothing. He then learns of Anakin’s mission of assassination and returns to Alderaan.
Anakin arrives first and goes to kill Amidala, but he cannot, for he loves her too much. Torn between his dark master and his love for her, he tries to escape, but Obi-Wan arrives and attacks. A tremendous, ultra-awesome light saber fight ensues, and Anakin is mortally wounded. His broken body is worked on day and night by top doctors, and beging fixing him up by grafting bionic accessories onto him. Including a breathing mask. His still-broken body is returned to Coruscant.
Meanwhile, Amidala tells Obi-Wan what she was about to tell Anakin; she’s pregnant! With twins! He tells her that no one must know and forbids her from leaving Organa’s home. He then embarks on a months-long journey to round up all the Jedi he can find.
The Jedi Council has decided to withdraw its support from Palpatine, but he isn’t concerned. Fewer than a hundred remain from the brutal Clone Wars, and he has a new ally; Darth Vader! Yup, Anakin’s now dressed in black and he goes all samurai on the Jedi, killing most of them. Yoda flees to parts unknown as Vader slays the remaining Jedi in action sequences of astonishing violence and coolness and Palpatine declares himself Emperor.
Obi-Wan gets back with a few dozen Jedi he rounded up, but it’s too late to save the Order. Vader and the Imperial Army, led by the evil Tarkin, slay them with ease. Obi-Wan gets into another wicked fight with Darth Vader, who can’t quite kill him but is clearly the better man. Then… Amidala arrives with a coterie of troops from her home planet and from all over! A tremendous battle is fought. Vader tries to kill Amidala and thinks her dead, but she survives in some miraculous fashion. Everyone good dies except for Obi-Wan, Amidala, who escapes, and Yoda, who’s somewhere.
Obi-Wan returns to Alderaan under the transparently assumed name of Ben Kenobi. The twins are born to Amidala and she and Obi-Wan agree that they must be separated. Leia stays with her; Obi-Wan returns to Tatooine, for some reason, and gives Luke to hsi uncle and aunt, but stays nearby to keep an eye on him.