We're not even going to tell you how it begins. You know how it begins: with that iconic yellow-fonted scroll of text and that epic, epic music.
But, when the text ends, the movie begins—smack dab in the middle of what is a truly epic space battle/operation led by Obi-Wan Kenobi, Anakin Skywalker, and his trusty R2-D2 droid. They're off to save Chancellor Palpatine from the dastardly clutches of General Grievous (if your name is Grievous, you know you're no good) and the Sith Lord, Count Dooku (if your name is Dooku, you—oh, wait. Dooku actually sounds pretty sweet).
Dooku challenges Anakin and Obi-Wan to a duel, but he gets decapitated for his effort. We cheer, but this weighs on Anakin's conscience. Grievous, however, escapes to fight another day.
Back on Coruscant, Padmé reveals to Anakin that she's pregnant with his child. This should be happy news … but instead of triggering a happy dance, Anakin has horrible, prophetic dreams about her dying in childbirth. Yeesh.
While Anakin is obsessing over what might happen to his lovely baby mama, his relationship with Chancellor Palpatine puts him in a position to spy for both sides of a growing conflict. Yup: Anakin is a double agent: Palpatine appoints him to the Jedi Council to be his "eyes and ears of the Republic," and then the Council asks Anakin to spy on the Chancellor in turn.
(Anakin is basically a galaxy-traveling James Bond.)
Meanwhile, Obi-Wan goes to Utapau in order to hunt down General Grievous. After a wicked cool chase scene involving a good-natured giant lizard, Obi-Wan kills Grievous with a blaster. Having an evil name doesn't pay, Grievous.
Meanwhile, Chancellor Palpatine reveals himself to Anakin as Lord Sidious (evil name alert). Anakin actually does the right thing and reports his evilness to Mace Windu. Mace goes to arrest Palpatine but ends up trying to kill him instead because he's too dangerous to leave alive.
Anakin intercedes (because, in a plot twist, Palpatine is the only guy who might be able to save Padmé from her certain death), and Mace ends up getting flung to his death by a deformed Lord Sidious. This marks a point of no return for Anakin, so he pledges himself to Lord Sidious, who then dubs him Darth Vader.
His first assignment as Lord Sidious' apprentice is to go to the Jedi Temple to kill everyone there, including the younglings. Meanwhile, Palpatine issues Order 66 (that's one 6 short of 666, y'all), which goes out to planets all over the galaxy and commands well-placed minions to betray and kill every Jedi. Yoda, Obi-Wan, and their non-Jedi ally Senator Bail Organa escape this treachery and unite to figure out what they should do.
Anakin gets sent to Mustafar, where the Separatist leaders have taken refuge, and kills everyone there as well. Anakin is taking to this evil thing like a fish to water. At the same time, Chancellor Palpatine announces to the Senate that he's going to be Emperor Palpatine from now on, to thunderous applause by everyone but a dismayed Senator Organa and Senator Padmé Amidala.
Padmé decides to go to Mustafar to confront Anakin about the rumors that he's turned to the dark side, and when she gets there, she realizes the rumors are 100 percent true. The lovers cry, and fight, and then Obi-Wan shows up … because he has stowed away on Padmé's ship.
Enough is enough: it's time to end this. Anakin thinks that his mentor and his wife have been having an affair, so he strangles his wife in anger using the Force. Oh, come on, Anakin. Stop being so evil. You were just a good guy.
Back in the Senate chambers, Yoda confronts Lord Sidious and they battle it out, using lightsabers and lightning bolts, and flinging Senate booths with the Force. We've never seen this little green Jedi master kick quite so much butt. The fight ends in a stalemate, with Yoda getting rescued by Senator Organa in the nick of time.
Back on Mustafar, Obi-Wan and Anakin battle amongst the raging, molten lava until Obi-Wan gains the high ground. In one swoop, he cuts off Anakin's legs and good arm, and he leaves him to die on the shores of the lava flow.
Obi-Wan rescues the unconscious Padmé before he leaves the volcanic planet, but it appears to be too late. She's dying. Before she goes, though, she delivers twins: Luke and Leia Skywalker. At the same time on another planet, after being rescued by Palpatine, Anakin is being transformed into the half-human, half-droid Darth Vader that we all know and love—er, loathe.
The movie ends with Darth Vader beginning to build his infamous Death Star, while Luke and Leia are being safely stashed with their new families, awaiting their next appearance in A New Hope.