Sam and Quorra are at the portal, Sam not wanting to go through, CLU approaches Kevin, and after a brief smackdown, pulls the disk from Flynn's back. We see a disk sever CLUs arm and another pick it up...it's Tron! He urges Kevin to go, but Kevin stands along side of him to fight.
...Battle, Battle, Battle...
Flynn falls down and Tron takes his disk from him. With Flynn begging him not to do it, Tron places Flynn's disk on his own back, thus taking on the identity of Flynn. Tron as Flynn now, re-assimilates with CLU causing the giant explosion, JUST as Quorra finally pushes Sam through the portal.
...Blah, Blah, Blah, Sunrise...
CREDITS
After-Credits-Scene: The bare grid, a wasteland, a lone figure, bathed in a white light emanating from his robes, calmly and assuredly limps to a small bare patch, and lays Tron's disk down. After pressing a few buttons, he walks away leaving the disk to grow a few blue computer bits on the top of it, like a digital flower emerging from the soil.