Awesomeness Is Volatile: Although Sento was able to fend off Night Rogue with KeyDragon's newfound power, it starts to overwhelm him once Night Rogue escapes, forcing Sento to manually remove the FullBottles to end his transformation.
The Bad Guy Wins: Despite Sento's and Ryuuga's best efforts, Faust was successful in stealing Pandora's Box from the Touto Institute of Advanced Matter Physics, Sento loses almost all of his FullBottles except for Lock, and Prime Minister Taizan Himuro suffered a stroke, leaving Gentoku as a replacement Prime Minister in his absence.
Clone by Conversion: Blood Stalk reveals himself to have this ability by changing Kawai's face to match Kuwata's face.
Foreshadowing: While fighting Blood Stalk, Ryuuga manages to work himself up to Hazard Level 3, which means he's finally able to use the Build Driver to transform. Something that he will do come next episode.
Similarly, when Sento confronts Stalk over supposedly being Katsuragi, Stalk never directly admits he is. He only doesn't deny it. In doing so and going on a Motive Rant about the Rider System being meant as a weapon to spill blood he tricks Sento into thinking that Katsuragi was a bloodthirsty monster. It was all a misdirection to make Sento despise Katsuragi to throw him off the trail of his true identity, a face-swapped Katsuragi.
The truth is that Stalk isn't exactly lying either. The Build system was devised by Katsuragi's father as a means to protect people from Stalk. The Rider system was Stalk's plan to destroy the Earth.
Hope Spot: Sento was starting to make a comeback starting with the debut of KeyDragon, but is ultimately reduced to one FullBottle and unsuccessful in reclaiming Pandora's Box.
Incredibly Obvious Bug: Averted, however Misora does find a transmitter under the stairs to the purifier machine. Given the circumstances, it's likely that Sawa put it there.
Lampshade Hanging: Ryuuga is dumbfounded that the only two FullBottles that Sento was able to use to fight back against Night Rogue after losing most of his FullBottles is another Best Match.
Shoo Out the Clowns: There's a noticeable lack of Souichi giving Ryuga an earful for exposing Misora to danger outside. Or Souichi doing much at all for this week. There's a reason why that is.