Order 66 and the Jedi Purge
Which means it’s finally time for Palpatine to execute Order 66, a secret command that causes an “inhibitor chip” inside every clone’s head to activate, turning them into Jedi killing machines. Revenge of the Sith proceeds to show the audience a montage of clones all over the galaxy slaughtering their Jedi generals. Meanwhile, Anakin leads a battalion of clone troopers into the Jedi Temple, where they kill every last Jedi in their path. Basically, the Jedi have been declared enemies of the Republic and must be executed on sight.
By the end of the montage, the only Jedi left seem to be Obi-Wan and Yoda. Discovering that Palpatine and Anakin are behind the Jedi Purge, they each head off to try and take down the Sith lords. Yoda confronts the newly-crowned Emperor Palpatine in his office, while Obi-Wan visits Padme to try and discover Anakin’s whereabouts. Obi-Wan tells her the truth about Anakin and she rushes off to find him, unknowingly leading the Jedi Master right to his former apprentice for a final showdown.
Obi-Wan vs. Anakin on Mustafar
Anakin is on Mustafar, the volcanic planet where the last remaining Separatist leaders are hiding from the new Empire. Palpatine, in his hooded Darth Sidious guise, gathered all of the clueless Separatists in one convenient place so that Anakin could show up and quickly bring an end to the Clone Wars.
By the time, Obi-Wan catches up with Anakin, the time for talking is over. The duel that was foretold all the way back in 1977 has begun. It’s one of the best action sequences in Star Wars movie history, as Anakin and Obi-Wan attempt to kill each other while also trying not to fall into a river of lava, so you should at least try to watch those scenes if you don’t have time to rewatch the whole Prequel Trilogy.
That said, if you really don’t feel like it, you at least already know who won this fight. After gaining the high ground, which is apparently check mate when it comes to lightsaber dueling, Obi-Wan begs Anakin to surrender. But when Anakin refuses, Obi-Wan is forced to cut his former apprentice down, slicing off the one human arm he has left as well as both of his legs.
It’s then that Obi-Wan says his final goodbye to Anakin, leaving him to burn to death in the fires of Mustafar.