I don't mind spoiling myself with this since everything we've seen is likely from the first or second episode.
214 REPUBLIC CITY, NIGHT
From their cables, Korra and Tenzin both spot something that causes them to stop fighting. The lava river ahead drops off in a tremendous lava fall.
SNAPPING AND METAL GROANS are heard as the main part of the collector starts to break away and move toward the lava fall. Tenzin looks around and sees a small floating platform making its way toward the tower.
Tenzin does a double back-flip and lands squarely on the floating platform. He immediately leans to one side and moves away from the tower.
Korra realizes he is doomed as the entire tower heads for the falls. In the distance he sees some POLAR BEAR DOGS. He swings back to the tower, climbs up and makes a running leap and miraculously lands on A POLAR BEAR DOG. The POLAR BEAR DOG is confused and GROWLS. The giant collector goes over the lava flow and disappears in the mist of sparks below.
Tenzin heads for the bank of the lava river, but Korra's POLAR BEAR DOG is faster. He catches up with his old Master.
Tenzin and Korra continue the airbending fight. They battle away, balancing on the tiny platform and puzzled Polar Bear Dog. Korra, standing on the Dog, approaches Tenzin on the work platform.
Tenzin: I have failed you, Korra. I was never able to teach you to think.
Korra and Tenzin confront each other on the lava river.
Korra: I should have known the Benders were plotting to take over . . .
Tenzin: From the Equalists!!! Korra, Amon is evil.
Korra: From the Benders point of view! From my point of view, the Benders are evil.
Tenzin: Well, then you are lost!
Korra: This is the end for you, My Master. I wish it were otherwise.
Korra jumps and flips onto TENZIN's platform. The fighting continues again until Tenzin jumps toward the safety of the black sandy edge of the lava river. He yells at Korra.
Tenzin: It's over, Korra. I have the high ground.
Korra: You underestimate my power!
Tenzin: Don't try it.
Korra follows, and Tenzin cuts his young apprentice at the knees, then cuts off her left arm in the blink of an eye. Korra tumbles down the embankment and rolls to a stop near the edge of the lava.
Korra struggles to pull herself up the embankment with her mechanical hand. Her thin leather glove has been burned off. She keeps sliding down in the black sand.
Tenzin: (continuing) . . . You were the Chosen One! It was said that you would, destroy the Equalists, not join them. It was you who would bring balance to the Four Nations, not leave it in Darkness.
Tenzin picks up Korra's airbending staff and begins to walk away. He stops and looks back.
Korra: I hate you!
Tenzin: You were my sister, Korra. I loved you.
Korra's clothing blows into the lava river and ignites. Suddenly Korra bursts into flames and starts SCREAMING.