The star skiff lands on Mustafar, and Padmé is greeted by a smiling Vader. Padmé reveals her conversation with Obi-Wan; he tells her it is only lies, though he doesn't deny the accusations. Padmé begs him to run away with her to raise their child in peace. Vader, mad with power, counters by telling Padmé that he has brought peace to the Republic. He then states his intention to overthrow Palpatine so the two of them can rule the galaxy together. Padmé, realizing how much Anakin has changed, backs away in horror and tearfully begs him to stop.
As she pleads, Vader sees his former master Obi-Wan emerge from the star skiff. He then accuses Padmé of lying about her love for him and that she brought his former master to kill him. Confused and enraged, he Force chokes his wife: Obi-Wan orders him to let her go, which Vader subsequently does only after he has throttled Padmé into unconsciousness. Vader accuses Obi-Wan of turning her against him but Obi-Wan retorts that Vader has done that himself through his anger, greed, and lust for power. Vader states that he had brought peace, justice, and freedom to his "new empire" and offers Obi-Wan a reprieve so long as he stands aside; however, Obi-Wan maintains his loyalty to democracy. Snarling that those not with him are his enemies, Vader draws his lightsaber; with no other choice, Obi-Wan prepares to battle his former pupil. The two then break out into a ferocious duel of Student vs. Master.
In the Chancellor's holding office within the Senate Building, Yoda confronts Darth Sidious. They engage in an intense battle using their lightsabers and Force abilities. The duel brings them to the Grand Convocation Chamber, the heart of the Senate. Sidious hurls Senate pods at Yoda, and the effort to avoid them exhausts the Jedi Master. Sidious then blasts Yoda with Force Lightning, but with the last of his strength the Jedi master repels it enough to send both himself and Sidious over the pod's edge. Palpatine is able to hold on to a senate pod, while Yoda falls to the senate floor leaving only his cloak. With Bail Organa's help, Yoda flees the Senate Building, intending to go into exile.
The epic duel between Obi-Wan and Vader leads them through the Separatist stronghold on Mustafar. A missed slash from Vader's lightsaber damages a computer console, resulting in the deactivation of the bunker's shields. The duel continues as the entire facility is bombarded by lava, and both combatants ride down a lava river atop a collection arm severed from the main structure. Kenobi and Vader leap from the arm and continue their intense duel atop floating platforms. Obi-Wan makes one last appeal to his old friend, trying to convince him of Palpatine's evil, but when Anakin shouts that the Jedi are evil from his perspective, Obi-Wan roars back "WELL THEN, YOU ARE LOST!", accepting his student, friend, and brother is beyond redemption.
Though on the defensive throughout the fight, Obi-Wan eventually gains the high ground by leaping to the safety of a black sand bank, and in doing so, the upper-hand. Obi-Wan warns Vader that he now holds an unassailable position and begs him, out of a last vestige of friendship, not to try and continue the fight, but Vader, enraged and bloodthirsty, snarls, "You underestimate my power!" He recklessly leaps up to meet his old Master. In the blink of an eye, Obi-Wan performs a lightning-fast maneuver, severing Vader's left arm and both of his legs at the knees. Vader tumbles down the embankment and rolls to a stop at the edge of the river of lava.
As Obi-Wan watches in despair, Vader, now only being kept alive by his hate, tries in vain to pull his crippled, smoking body up the embankment with his remaining mechanical hand. Obi-Wan is overcome with disgust and sadness at the pitiful fate of "The Chosen One", and is reduced nearly to tears. He angrily shouts that Anakin has failed in his life and destiny, and has hurt all those who loved him, including Obi-Wan himself. Vader, now completely helpless and consumed by his dark-side emotions, can only scream in pain and powerless rage, cursing his former friend and Master. The lava below Vader then catches his leg, setting him on fire and horribly burning and disfiguring him. Obi-Wan, not willing to help his former pupil and best friend, or watch him die or kill him in cold blood, picks up Anakin's lightsaber and walks away. He returns to the star skiff and leaves the planet with the badly injured Padmé and her droids.