- Written by Shogo MutoDirected by Kyohei Yamaguchi
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... And then something crazy happened! The shock of it has ruined our Christmas mood! We were going to do the introduction while singing Jingle Bells, too... Just start episode 16 already!
- Early-Bird Cameo: Near the end of the episode, we see the new Kamen Rider Grease in action. We don't know who he is however.
- Foreshadowing: Sento tries to build a Sclash Driver in his funk before Ryuga tries to pick a fight with Sento. As seen from the trailer, this will be the same Driver that Ryuga will use in the next episode.
- From Bad to Worse: Even though this happens at the end of the episode, the fact that Hokuto now has Smash and the fact that there is a new Kamen Rider around shows that Nanba Heavy Industries has already branched out to the other two factions for their war effort.
- Get A Hold Of Yourself Man: Ryuga picks a fight with Sento knowing that although it changes nothing, he felt like he needed to beat some sense into Sento.
- Heroic BSoD: Sento doesn’t take the revelation of his true identity well, and even when he is forced into a fight, he either shows a token amount of resistance, or becomes so engrossed in his thoughts that he performs poorly. This sticks with him until near the end of the episode when Ryuga convinces him to be a hero again.
- I Am What I Am: Despite the fact that he almost became a Death Seeker to try and atone for his past actions as Takumi, Sento declares that he is the narcissistic, conceited hero of justice in an amazing He's Back! moment after remembering all the good he has done as Build.
- Klingon Promotion: Taizan Himuro is attacked by Blood Stalk, right as he was to re-assume his position, meaning that Gentoku is now the official Prime Minister of Touto in the wake of Taizan’s returned hospitalization.
- Laser-Guided Amnesia: This is revealed to be one of Soichi’s other powers that he acquired from the Mars expedition.
- The Bad Guy Wins: Gentoku and Nanba get exactly what they'd been planning to get the entire series: all out war between the divided Japan.
- This Means War!: Gentoku basically tells the other two Prime Ministers to drop their facades and convinces them to go to war for possession over Pandora’s Box, with their reactions implying that they accept his declaration.
- Title Theme Drop: Right as Sento remembers about all the times he did good as Build, the music kicks in to emphasize that he is a hero and “the one.”
- Voluntary Shapeshifting: A random Hokuto soldier apparently has the ability to transform into a Smash at will while retaining his consciousness, though the tone expressed by his comrade implies he’s not the only one with this ability.
- Wham Shot: There’s a new Kamen Rider in Japan, and he's wearing a Sclash Driver.
- You Are Better Than You Think You Are: Two examples:
- Misora finds Sento after his brief skirmish with Ryuga, and tells him that although she doesn't know much about Sento, Misora believes that Sento Kiryu is a narcissistic, conceited hero of justice to try and cheer him out of his depression.
- When Ryuga intervenes to fight off the new Smash that gave Sento trouble, Ryuga tells Sento that even if he was responsible for the Smash and ruined lives, he also created the Build Driver that has the potential to save lives, which made Sento remember everything he did up to this point as Build.
