- Written by Shogo MutoDirected by Satoshi Morota
After the reveal of the Pandora's Box panel beneath nascita, Isurugi reveals his past to Sento and Ryuuga: he was the astronaut that found the cube on Mars, as well as the one who touched it and created the Skywall... though he can't explain why he did it. At one point, Faust kidnapped Misora to utilize her powers, and Souichi infiltrated the organization by offering the two panels that flew out of the Box. He soon took the chance and fled with his daughter, but not without taking back one of the panels and going into hiding. When he found Sento a year back, he knew that this was a chance to put a kibosh on Faust's plans and coerced him to become Build.
But this isn't the only revealed past, as Sawa comes in, saying that she found someone that knew Sento! That someone is Tatsuya, Sento's roommate as well as... a member of his punk band?! While Kiryu reels from the shock of his past, Blood Stalk plans an elaborate scheme to test Build mettle.
- Curb-Stomp Battle: Build doesn't even need to use a Finishing Move to defeat the Press Smash/Tatsuya.
- Easily Forgiven: Despite the fact that not every detail is clarified, Sento is willing to forgive Souichi for his secrecy since he believes a truly terrible person would not be willing to sell a disgusting coffee blend named after a pun.
- Evil All Along: Discussed in episode. While Souichi doesn't explain what role he had in Faust, he and Misora quickly debunk any notion that he is a bad person with the fact that he rescued Misora from Faust.
- Evil Laugh: Blood Stalk has one walking away from his first fight with Sento.
- Hard Head: Headbutting Tatsuya twice has no effect on Ryuuga whatsoever.
- Heroic BSoD: Sento has a mild one after learning about his past as Satou Tarou, though he quickly recovers when Misora tells him about a new Bottle.
- Hostage For Macguffin: In order to gain Faust's trust to even find Misora, Souichi stole the two Pandora's Box panels from the Touto Government.
- Kleptomaniac Hero: A very downplayed example, but the fact that Souichi decided to steal whatever FullBottles he could along with one of the Pandora's Box panels when escaping with Misora at least makes him this trope in passing.
- Manchild: Part of Sento's act in the episode. He lets out something like highpitched WOO-HOO when rushing to pick up a new bottle, flips on the floor like child throwing tantrum and nearly pokes Ryuuga with Ninpotou to test it out.
- No-Sell: The first time Build tries to use his Rider Kick against Blood Stalk, the cobra man stops it with one hand.
- Unwitting Instigator of Doom: Misora's offhanded comment to Ryuga about Smashes returning to their base to recover leads him to turning Tatsuya back into the Press Smash just for a lead to Faust's base.
- Use Your Head: Ryuuga headbutts Tatsuya twice when he tried to bite more than he can chew when he antagonized Banjou.
- Villain Takes an Interest: After Build manages to hurt him in their second encounter, Stalk gleefuly admits that he's "everything that he hoped for" and leaves.
