Ashinaka High School is known for its unique setting: the entire campus is built on an island. Yashiro Isana, aka Shiro, is having lunch with a cat on one of the school rooftops. After his lunch break, Shiro goes on an errand for his classmate Kukuri to prepare for their upcoming school festival, only to be chased by some intimidating-looking men.

Shiro begins to question his own identity after realizing there is an inconsistency between what he remembers and what actually happened. Meanwhile, Lieutenant Seri Awashima of SCEPTER 4 makes a visit to the bar owned by Izumo Kusanagi, where she learns more about the history behind HOMRA and the strong bond they share.