Ashi and Jack face their demons in Season 5, episode 7 of 'Samurai Jack'

Jack's real enemy isn't the demon Aku, but the demonic anger within himself. 

Samurai Jack has always depended on the moral goodness of its title character. If Jack weren't committed to righteousness and protecting the innocent, we viewers would not care for him, and he would not have rallied so many to his side. And if his skillful commitment to these virtues did not make him such a threat, then Aku and his minions would not care to oppose him.

But what if Jack were not worthy? As it turns out, this is more than idle speculation. For despite the journey we have seen Jack undergo, not just in this season, but in seasons 1-4, he is not perfect. This episode focuses on one of Jack's imperfections: the (understandable) frustration and anger accumulated from his many setbacks and disappointments. For Jack, in this episode, this is mostly a mental struggle. Meanwhile, Ashi demonstrates it physically, proving to herself and to the world that Jack's quest is worthy, and he is worthy of his quest.

As promised last week, the primary narrative drive of this week's episode is Jack's quest to retrieve his sword. We saw earlier this season that he had lost it, and how its loss had changed him, but we did not see exactly how he had lost it. But this episode opens with a flashback telling us how. At some point in the past, Jack discovered a time portal, but Aku (voiced by Greg Baldwin, who sounds basically indistinguishable from Aku in this flashback) destroyed it, and informs him it was the last one in existence. In his frustration, Jack ruthlessly slaughters some beasts Aku had summoned to distract him ("I know better than to mess with that sword!" Aku says as he flees). After Jack graphically kills the beasts, he realizes that they were merely transformed versions of the friendly lamb-like creatures who had led him to the time portal in the first place. In the resulting despair and confusion, Jack's sword plummets down the hole the time portal once occupied.

When we return to the present time, Jack and Ashi return to this place to search for the sword. But a glance at one of the skulls of the creatures Jack killed, a symbol of his wrath, makes him realize this is a fruitless search. "I did not lose the sword. But the sword left me."* In his anger, it seems, Jack became unworthy of the weapon. Thus, Jack must retreat inward, and sends himself, through meditation, on a quiet vision quest (that has some of the best animation and music of the show yet) to restore his proper mental state.

Jack, trippin' on his quest.

His lack of balance manifests in the boisterous, quasi-demonic, distorted version of himself that magnifies any negative feelings he might have. Early in the season, it magnified his despair; later, his guilt; and, now that he has returned to his quest, his frustration at being unable to complete it. This part of Jack wants the easy way out; when Jack's spirit guide tells him that "it is not for me to show you your path," demon-Jack demands it, especially "after everything we've been through...the death...the loss...the suffering!" But the real Jack discards these feelings of wrath and impatience, emerging from his inner ordeal as a triumphant restoration of his old self.**

Jack's back. 

Most of Jack's scenes in this episode take place in his own head; he mostly sits completely still. This leaves Ashi to guard him, and boy, does she have her work cut out for her, with loud and violent tasks that contrast directly with Jack's quiet but equally important inner quest. First, she fends off an entire army of orc-like goons come to kill Jack. But, more challenging, and more significant, is the attempt on Jack's life by Ashi's mother (voiced by Grey DeLisle, also the voice of Vicki, the evil babysitter of Timmy Turner in Fairly Oddparents, who is voiced by...Tara Strong, the voice of Ashi). The Mother trained Ashi from birth to kill Jack, and provides the first real test of Ashi's commitment to her new moral universe, as well as a significant physical challenge, as she is a formidable foe. And though the Mother tempts and guilts her back to her old life, Ashi proves her change of heart by valiantly defending Jack and killing her. And with her Mother gone, Ashi has now fully rejected the evil she once served.

Ashi, dealing with some mommy issues.

This episode ends with Jack restored to his old self, Ashi wholly committed to his cause, and both of them ready to move onto their next (and final) goal: defeating Aku. I was ever-so-slightly disappointed that this episode was structurally very similar to the previous one (Jack has a mental struggle and emerges from it, Ashi is the main focus in the physical world, and it ends with them announcing their next step: last week, finding Jack's sword; this week, defeating Aku). But I'd be lying if I didn't smile when Jack was back in his samurai garb, wielding the sword he has now once again proven himself worthy to wield. It is only a matter of time before he gets a chance to use it again.

And I, for one, can't wait.

*Jack's sword, in this case, seems to function a bit like Thor's hammer: If Jack is unworthy of wielding it, then he can't use it. We saw a trace of this in the original run episode "Jack and the Zombies," when Aku attempted to use Jack's sword to kill Jack, but could not, for the sword could only be used for good.
**I guess this means we'll never get to see Jack look like this: