An experimental X-series robot was given emotions as an experiment and therefore was the only one of it's kind to survive intact. It retired after taking a shine to sweet thing Lulu, but Aku forces it to take on one more job: hunt down Samurai Jack.
The young emperor is helpless against the evil of Aku. But the ancient gods Odin, Ra and Rama combine forces to forge a sword for him representing all that is good: the only weapon that can hurt Aku.