Special episode included in the Blu-ray Box of Ano Natsu de Matteru.

With the arrival of summer and the school break, Chizuru, Kota, and the rest of their friends are lazily enjoying the warm weather. Chizuru flirts with Kota, Kota gets embarrassed, and Nozomu gets in the way. Tayura pines after Akane, Akane rebuffs Tayura, and a sexy good time ensues. The status quo isn't quite the same since the fox snared her prey, but the timeless boy-meets-girl tale lingers on. (Source: Media Blasters)

A runaway vehicle driven by Izumi Yasaka, en route to the Public Safety Bureau building, is reported shortly before it crashes into the building. Izumi is a counselor who recently ran away while working at a latent criminal isolation and rehabilitation facility known as Sanctuary. Before Inspectors Mika Shimotsuki and Akane Tsunemori get to interrogate the suspect, a sudden request is issued from the facility to promptly bring Izumi back. Interpreted as a direct order from the Chief and the board at Sanctuary, the Inspectors obey, but insist that Izumi be escorted back personally. Tsunemori intends to investigate further with the rest of the team at the Bureau. Now Inspector Shimotsuki has finally been given the opportunity she had been waiting for—to be the primary investigator on an important case. This case follows Shimotsuki and her team of Enforcers as they uncover the secrets of Sanctuary.

At Arkham Asylum, Batman clashes with Gorilla Grodd while trying to destroy Grodd's newest invention: the Quake Engine. Amidst the chaos, the engine activates, and the entire asylum suddenly vanishes into thin air—Batman along with it. Once the dust settles, Batman finds himself standing in the middle of an unfamiliar road. He soon realizes that he is no longer in Gotham City—but instead in Feudal Japan!

The amazement quickly fades as samurai descend upon him, seeking to take his life. Later on, Batman's investigation leads him to discover that the one responsible for the kill order is none other than his archnemesis, the Joker. Following an encounter with Catwoman, he learns that the asylum's criminals had teleported there two years earlier—each ruling over a piece of Japan with Joker being the dominant warlord.

In order to return home, Batman must reactivate the Quake Engine, situated in the remnants of the asylum, now known as Arkham Castle. However, there is one problem: aside from his gadget belt, the arsenal that was once at his fingertips is all but gone. Ninja Batman follows the Dark Knight as he traverses a strange new environment, facing off against familiar foes in a bid to return to Gotham in one piece.