Chapter 18 is titled "Always Shop Responsibly".
Daruma engages the two thieves aboard his Key Holder. Though Ryu's skill with his Samurai Soul is great enough for him to nearly escape, Daruma comes out on top. At the same time, Hachimaru's inexperience handling money puts his master in a tight spot. Fortunately, Kotsuga points them towards a potentially massive windfall, which could lead to another Key to Pandora's Box...
Hachimaru asks Daruma about the two intruders aboard the holder, and he warns that the Samurai they face is a powerful one. He then demands that the intruders state their business, and Kotsuga declares their intent to steal his belongings, confirming to Daruma that they are ronin thieves. Surprisingly, Ryu turns his blade on Kotsuga, who stops Ryu an instant before being cut down. Kotsuga asks why he is attacking his partner, and Ryu tells him that he is simply following his order to kill everyone but Ann. This sparks an argument: Kotsuga is angry at having to state the obvious, and Ryu points out that Kotsuga has been forgetting that he can only understand completely literal statements. Hachimaru begins to doubt that the duo is formidable at all, while Ann finds their banter to be sweet.
Daruma instructs Hachimaru to watch over Ann while he takes the ronin down. Kotsuga properly sics Ryu on Daruma, but Ryu stops right before they clash to ask if Daruma is a male or female cat. He replies that he's a human male, so Ryu the begins attacking. He then pulls a second Samurai Soul out and connects it to the bottom end of his Handle Bone to make a double bladed saber. The ability to shape a Samurai Soul that way shocks Hachimaru. The new blade nearly hits Daruma, but he manages to form armor around himself using the turtle Holder, which he explains is his Key Holder. He then uses the ship itself to trap Kotsuga, but Ryu shapes his Samurai Soul into a scythe to cut his partner free, then forms a drill to escape through the Holder's head.
Daruma follows the ronin to continue the fight, but as he does, his AI alerts him that his credit payment for the drink he ordered at the bar fell through. Daruma is shocked, as he knows he has money saved up, so he has his transaction records pulled. As Ryu merges his two Soul blades into a single large blade, Hachimaru watches the battle from inside the ship, impressed that Ryu can use his Samurai Soul to fly and fend off his master and carry his partner. Daruma learns the cause of the error: in addition to the ¥35,000 used for Ann's groceries and Hachimaru's clothes, his student spent ¥17,000,000 at a textile store. At that moment, the bartender, who was looking for Daruma, arrives and demands he pays for his drink.
Hachimaru then contacts Daruma, worried because he is fighting while distracted, but he is actually infuriated that Hachimaru spent so much. Yelling at his student, his rage is so great that his shout becomes a wave of force severe enough to destroy his own armor, blow the two ronin away, and rock the entire spaceport. Once he calms down, Daruma scolds Hachimaru, who thought the item he bought was ¥1700 instead of ¥17,000,000. Hachimaru apologizes, but Daruma lets up, knowing Hachimaru knows little about how the world works. He asks about his purchase - a roll of gold embroidered fabric - and learns that it was a present for Ann. He explains that he wanted to replace Ann's current sash, which she tore to help bandage his father during the fight with Ata. Though grateful for the gesture, she tells him she is happy with her damaged sash and offers to go with him to return the expensive fabric.
While they talk, Kotsuga - who was detained along with Ryu - offers to drop some information that could help with their money problem. Though wary, Daruma agrees to hear him out. Kotsuga explains that on a certain planet, a festival for unruly people is held, along with an anything-goes fighting event. The winner of the battle royale gets to fight the festival's sponsor, and if they defeat them, they earn ¥1,000,000,000. However, the sponsor is a Samurai claiming to have a Key to a legendary box, and is said to be able to split a planet open with a one swing of a sword. Ann wonders if they could be a Samurai needed to open Pandora's Box. Interested in settling their money problems and finding a potential ally, Hachimaru asks to be allowed to fight in the festival. Daruma agrees without hesitation, impressed by his pupil's line of thought. They then prepare to go to the site of the festival: a planet carved open in several places.