Through Sana, Yukina sees a memory of Natsuki and Aya ten years ago, and then dreams of a happier world she shares with Kojou, Sayaka, and their friends.

Kojo and the others go to rescue Asagi, who was imprisoned on the 0th floor of the Keystone Gate. Kojo is worried about Himeragi's poor condition, but Himeragi goes with Kojo nonetheless. What was waiting for Kojo and Himeragi on the 0th floor?