Satsuki, a boy high school boy and his childhood friend are riding in an underground train until it suddenly stops. Upon closer inspection the denizens of the train discover a mysterious beast! Panic sets in as the passengers struggle to survive the beast and escape to the surface.

This is again the 3rd Continuation of the series "Mayonaka no Jewel!" (Source: MU)
Includes the one shots "A Transient Mermaid In Love" and "Kake x Ochi."