"Episode XLIII: The Aku Infection" is the forty-third episode in the fourth season of Samurai Jack. This episode features Jack becoming infected with Aku's sickness and being slowly taken over by him.
Jack is battling another army of Beetle Drones in the snowy mountains. After dispatching the first wave, Aku, who has caught a cold and is very sick, sends the next wave. During the battle Jack rides a part of one of the drones and charges at Aku. Aku coughs and unwittingly causes Jack to be infected with a piece of himself before leaving Jack saying "No, I will not fight you today. I don't feel good". Jack continues on his journey but struggles to cope with the extremely cold temperatures of the mountains. He begins to hallucinate and catches a glimpse of his mother and father before passing out. A pair of miners find Jack and bring him to their tent, giving him some food to strengthen him. They explain that they have been mining the mountains and found three precious gems, needing just one more to retire. Jack promises to keep their secret in return for saving him as they notice some black marks on his hand.
The next morning Jack parts company with the two, despite their concerns. Jack soon bristles with the cold, only to discover that he has stolen the gems from the miners. Soon he finds a hiker trapped over the edge. He at first tries to rescue him but starts arguing with himself over whether he should help him out of kindness or get a reward in return. It gets to a point where the hiker suggests to Jack that he wishes to stay where he is.
As Jack continues his journey he continues to cough and struggle to breathe as more of his body is covered in darkness, causing him to scratch more. His right arm starts to continually flick out his sword and put it back in. Jack soon reaches a village, as a dog barks at him and children keep their distance. The final straw comes when Jack bumps into a robot and demands an apology before slicing the robot with his sword, only to stop when its grease splats onto his face. He runs to a barrel to find that his infection is taking control of him.
Jack desperately seeks out a cure before Aku's essence transforms him into the monstrosity himself and decides to seek the Lizard Monks, which his infection also decides to seek. Stealing a tent and covering himself with it, he soon reaches the hidden temple. He is welcomed into the temple and brought before Master Ning, the leader of the monks. Master Ning explains that the Pool of Convergence, which they use to study the infinite facets of reality, may be used to send Jack home and asks what Jack seeks. Jack sheds a tear before his infection takes full control of his body and proclaims his intention to destroy the Pool. Ning calls his brothers to fight back. Jack-Aku uses Jack's sword to fight off the monks, who fight valiantly, but are ultimately defeated. Jack-Aku is just about to destroy it but a last minute resistance attempts to stop him. Jack-Aku is overwhelmed and destroys the portal as Master Ning subdues him. As he loses consciousness, Jack begs for forgiveness.
Master Ning has Jack suspended over the Pool performing prayers and rituals, but informs him that it will take Jack's own will to overcome this evil. Jack says he cannot fight it alone as his infection takes over and threatens to devour every Lizard Monk as soon as he consumes the Samurai. Jack's inner spirit, showing his childhood self, is comforted by his mother and father and is shown a fortress made by the blessing of everyone he has saved and the many allies he has made on his journey. Given a small bow and arrow with a heart-shaped tip, Jack climbs the stairs and fires the arrow into the sky, creating a tower of light, turning Jack into his full adult self donned in samurai armor. He takes the beam and wields it as sword to strike at the darkness around him, with it turning into images of Aku. Jack cuts through every one of them, purging the infection.
Jack asks about his actions that caused so much trouble, Master Ning informs Jack that although the Pool is destroyed, the Nexus is still intact and in time they will rebuild it. Ning explains that they feel that they owe him their lives for defeating the infection declaring "All debts are paid". Jack says "all but one", and returns the stolen gems to the Miners.
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- Casting by
- When Jack accidentally swallow Aku's infection, Aku says "I don't feel good." It makes the reference of Spider-Man/Peter Parker's quote "I don't feel so good." from Avengers: Infinity War which fans believe and think that both Aku and Spider-Man/Peter Parker share the same quote as a joke and makes it a popular meme.
- The way in which Jack transforms into Jack-Aku is similar to the transformation made by Ashi in Episode C.
- Two of Jack's teachers, the English Archer and the African Chief appeared in his vision as those who were once helped by Jack. Given that the English Archer are based on Robin Hood, it's likely that Jack has helped the Merry Men overthrew the tyrannical Prince John.