- Written by Shogo MutoDirected by Kyohei Yamaguchi
Sento: The brilliant physicist and Kamen Rider Build, Kiryu Sento, is investigating the murder of scientist Katsuragi Takumi in order to prove Banjou Ryuuga's innocence. Their search unearths Katsuragi's involvement in the founding of Faust, and Sento obtains the data that was being held by Katsuragi's mother.
Ryuuga: Hey..."Kiryu Sento" is what the cafe owner named you, right?
Souichi: Yep! "Sen" as the character for tank, and "To," as in rabbit.
Sento: You just named me Rabbit Tank!
Ryuuga: Then, how about "Kiryu" then?
Souichi: I was on the way to my barber Kiryu when I found him.
Sento: THAT'S the reason?!
Souichi: Let's just start episode 9 already.
- The Atoner: While Ryuuga and Misora are alone, Misora tells Ryuuga that Sento regrets being unable to save Kasumi from her death and part of why he built the Cross Dragon was to help Ryuuga find his way.
- Cliffhanger: By the end of the episode, Blood Stalk leads a squad of Guardians to steal the Pandora's Box, and Sento is too far from the lab.
- Clingy Macguffin: The strange bracelet that Misora wears cannot be removed, but it allowed her to purify Nebula Gas essences into FullBottles, which made her a target for Faust despite the fact the process took about a week.
- Deep Sleep: While she may be an Ill Girl, Misora essentially went into a coma for seven years after the Sky Wall incident, and woke up with the bracelet already on her. And as she adds on, being used by Faust to make FullBottles put her in a week long coma for each bottle, but the machine Sento made in the lab cuts it down to just exhaustion.
- Dissonant Serenity: Misora, again. She talks about her time in Faust's captivity with much more calmness then she should by any means. Ryuuga calls her out on this. Her encounter with Night Rogue shows that this is probably coping mechanism as being directly reminded of the trauma strains her so much she passes out.
- Driven to Suicide: Shingo Kuwata kills himself with a Nebula Gas bottle before Sento has a chance to interrogate for answers.
- Lured into a Trap: Sento tries to defeat Shingo's Smash form to prevent a rampage, only to find out afterwards that his absence allowed Blood Stalk to waltz into the laboratory unopposed.
- The Mole: While it was revealed since Episode 7 that Faust has been a big conspiracy plot, it runs deeper than Sento realized with two of his co-workers, Shingo Kuwata and Eita Kawai, being willing accomplices to Faust.
- As it turns out in the end, Sawa has been working with Nanba Heavy Industries and has given Juzaburo photos of Build's recent Best Matches.
- Power Levels: While Hazard Levels have been talked about before, Sento goes more in depth on how they work;
- Hazard Level 1: A person who becomes a Smash Monster, but dies immediately after its essence is removed.
- Hazard Level 2: A person who becomes a Smash Monster, but still stays alive after its essence is removed (although they do lose a bit of memory).
- Hazard Level 3: At this level, the subject is able to become a Kamen Rider.
- Real Men Wear Pink: The shirt Ryuuga wears when talking to Misora is baby pink with obligatory dragon up front.
- Speaking Like Totally Teen: Souichi randomly starts speaking like this and claims a kid at one of his part-time jobs taught him slang.
- The Reveal: Project Build's origins, Sento's namesake, and the Wolf and Smartphone FullBottle Best Match are revealed in this episode, though the latter isn't used outside of being referenced.
- Villainous Rescue: Blood Stalk's appearance allows Ryuuga to escape with Misora long enough to be discovered and rescued by Sento.
- Wham Line: Takumi Katsuragi is still alive.
- Wham Shot: The person who shows Juzaburo Nanba the Build photos is Sawa!