Terror! The Monster That Lurks Underground

Petty thefts keep plaguing the temporary lodgings where the Fourth Division has camped ever since the Muramasa crisis. Tea leaves, rice, bean paste, and even Shunsui's private sake vanish one after another, and when the coop's chickens disappear too, the only clue is a blurred surveillance frame of a hooded shape snatching a bird. Because the footprints lead into the underground waterway, the leadership suspects one of the Tōjū is responsible.

Shunsui and Ukitake put together a small team of Nanao, Hanatarō, Ikkaku, and the manifested Hōzukimaru, tasking them with tracking the intruder while the research institute maps the tunnels in real time. Nervous ghost stories and dripping ceilings set the mood before the group stumbles into the raider's actual lair.

The party springs a string of rigged traps, gets swept far off course by a delayed flood, and is ambushed by a tentacled Tōjū that hauls Ikkaku underwater. Trailing the smell of cooking, they find a hidden camp stocked with every missing item and corner a cloaked figure who does nothing but apologize. He proves to be Hisagomaru, the spirit of Hanatarō's own healing blade, so changed in appearance that his partner barely recognizes him.

With Hanatarō having left his sword at the barracks, Hisagomaru grabs the abandoned weapon, drives it into Ikkaku to charge its gauge, then unfolds a chest cannon and releases Akeiro Hisagomaru to obliterate the Tōjū. Nanao afterward reveals that Hisagomaru, unable to revert to sword form like the other freed spirits, had convinced himself Hanatarō hated him and retreated to live as a hermit in the tunnels.

This is an anime-original installment within the Beast Swords campaign, adapting no manga chapters. Hōzukimaru activates his bankai Ryūmon Hōzukimaru, while Nanao contributes with Hadō #31 Shakkahō. The two partners promise to stay together, though a closing gag strands them on separate paths once more. The illustrated picture book segment shows a crowd of Shinigami and their spirits occupying Ichigo's bedroom, which they have declared their real-world headquarters.