"Dead Mount Death Play - Episode 18" begins with Inspector Tsubaki Iwanome facing the consequences of his entanglement in a corruption network, which was exposed through his interactions with the mysterious Solitaire and the grocer. This leads to a confrontation with Police Chief Habaki, who traded his daughter for his position, highlighting the depth of his ambition and ruthlessness.

Meanwhile, the narrative shifts to a flashback involving Xiaoyu and Majiri Agakura, where Majiri offers Xiaoyu a place in her family. This offer is rooted in their shared past and the violent world they inhabit, illustrating the complicated relationships between the characters.

The episode takes a mysterious turn when the Fire-Breathing Bug appears. Arase attempts to understand the Bug's actions, leading to a discussion about the possibility of hypnosis.

Later in the story, the Agakura siblings, Majiri and Momoya, encounter Misaki, who has been transformed into a "vampire larva" by Polka. The Agakuras recognize Misaki's potential and invite her to join their family.

At the climax of the episode, Chief Commissioner Habaki is confronted by his daughter Soara, whom he had subjected to experiments. Now known as Arahabaki and holding a rank above her father, Soara expresses her desire for revenge, resulting in a dramatic and emotional confrontation.

The episode concludes with Polka being approached by Sayo Shinoyama, who requests his services.