"Episode XL: Jack vs. the Ninja" is the fortieth episode in the fourth season of Samurai Jack. The episode features Aku sending the Ninja, a deadly assassin, to kill Jack, who must use his own martial arts to survive.
As the episode opens, the Ninja silently infiltrates a large Japanese temple where Jack is at prayer. The ninja sneaks up behind Jack and, before the Samurai can react, slices him in two. It is revealed that this is not really Jack but a robotic double and this whole thing has been a training exercise. Aku appears and, congratulating the Ninja for passing the test, sends him out to kill the real Jack.
Jack travels through a dense jungle, all the while being stalked by the ninja. Just before the ninja can strike, a boy races through with a giant, robotic, lobster chasing him. Jack rushes to the boy's aid and defeats two of the beasts. The boy tells Jack that more of the lobsters are attacking his village and Jack has the boy lead him there. The ninja is loathe to leave the cover of the jungle but eventually follows. Jack defeats them all while the Ninja watches. After the battle, Jack discovers that the ninja has taken the boy to a nearby ruined tower.
In the tower Jack finds the boy and frees him, narrowly avoiding a strike from the ninja. The ninja begins using hit-and-run tactics; hiding in the shadows, striking, then retreating to the darkness. Jack knows of the ninja's technique and reveals that he has been trained to blend with the light, just as the Ninja can blend with the dark. The two warriors fight from shadow to light and back again however, with the sun going down, Jack quickly runs out of concealment but reflects the waning sunlight into the Ninja's eyes and destroys him with a well-tossed sword. The episode ends with the boy running to Jack, hugging him as a sign of gratitude, as the sun fully sets.
- Casting by
- Collette Sunderman
- This episode is sometimes erroneously referred to as "Samurai versus Ninja" in fan circles.
- The same scream is reused three times for the boy while he's in the jungle, with the first fading out.