- Written by Shogo MutoDirected by Shojiro Nakazawa
Sento: Kamen Rider Build, the brilliant physicist Kiryu Sento, is pursuing the truth about the murder of Katsuragi Takumi to prove Banjou Ryuga's innocence. When danger looms and Pandora's Box and the bottles are stolen, Banjou finally transforms into a Kamen Rider.
Ryuga: Now I'm a Kamen Rider too!
Sento: So what name do you want?
Ryuga: We're deciding it here?
Sento: My personal favorite is Kamen Rider Musclebrain.
Ryuga: I wanna know how that got to be your favorite.
Sento: Oh yeah? Well, we'll make the official decision in episode 12.
- Bodyguard Betrayal: Utsumi was forced to pretend that this was the case in order to deflect any suspicion of Gentoku being Night Rogue.
- Cliffhanger: The episode ends with Nabeshima calling Sento, telling him that his memory has returned and to repay a debt to Sento, mentions who gave him the order to frame Ryuuga offscreen.
- Deceased Fall-Guy Gambit: To keep deniability about Faust's connections to Nanba Heavy Industries and Touto's government, Utsumi was set up to be this. Now everyone believes he is/was Night Rouge, save for our heroes.
- Disney Villain Death: Nariaki Utsumi was shot in the shoulder and the force pushed him over the edge of a bridge, plummeting to his death in the waters below.
- Easily Forgiven: Played With as while the nascita crew don't necessarily believe that Sawa is telling the whole truth, they choose to forgive her and let her stay with them as a means for redemption and to clear the reputations of Build and Ryuuga. This brings her to tears.
- Jacob Marley Warning: Utsumi tries to convince Sento to not end up the same way as he himself has, but is killed before he can say anything else.
- Heel Realization: After Utsumi's cover is blown as Night Rogue, he tries to claim that he wouldn't have become Night Rogue if there was any other way to have Japan be unified and recover, only to stop in the middle of his words and attempt suicide by Nebula essence. When that failed, he chose to instead run away.
- Never Found the Body: Despite being shot and falling from the bridge, there is a distinct lack of Utsumi when Sento looks over the edge for him, and no one says anything about there being a body afterwards. Time will tell if it's played straight or if
He's Just Hiding. - "Not So Different" Remark: Utsumi claims that he and Sento are birds of a feather in that they are both trapped in their situations after revealing the hidden third basement floor of the Nanba Heavy Industries Institute of Integrated Science.
- The Reveal: The episode intro teased that the official name for Ryuuga's Rider form would be revealed at some point in the episode, and it does at the very end, when Sento tells Ryuuga that Ryuuga can't be Build because Ryuuga is Cross-Z.
- Stolen Good, Returned Better: Played With. Sento not only takes back his bottles this episode, but he nabs Faust's Box panel, and the Densha Fullbottle
- The Teaser: The episode opens with a cold open that lets Sawa explain part of her situation and leads into the main plot of the episode, eschewing the opening.
- You Killed My Father: Sawa's reason for joining Nanba. Her father was a group of researchers working to study the box for either Nanba or Faust but after a lab explosion, all ten were killed to cover it up. She wants to expose the truth of the matter.
- Full Bottles needed for Pandora's Box Activation: 3
