Vash confronts Knives and they remember their past together.  They are both forms of the Plants used in the machinery on the planet.  Long ago Knives decided that people show be destroyed leaving his kind alone while Vash took Rem's ideas to heart and tried to make a paradise for everyone.  Knives and Vash's duel with their weapons that Knives made from other Plants and the fate of the world is at stake.