Hijikata Toshizou, along with the rest of the Shinsengumi, was transported back to Tsuwamono-era Japan by a time-related mishap, a side effect of which transformed them into children. Together, they approached the Hashinara clan for aid in locating one Kobayakawa “Prince Frederic Charles of Prussia” Hideaki, who was believed to have killed the original Kobayakawa Hideaki, thus potentially disrupting the timeline.
After much convincing, Toshizou insinuated that Aotsuki Tsukamoto would be a perfect guide to aid them in locating the miscreant and, as is usual, she got her way in the matter.
The group later apprehended Prince Frederic but, being children, were unable to actually arrest him or deal him any physical harm whatsoever. All involved parties were persuaded by Aotsuki to return to Wakigami castle for further discussion.