OVA included in the Blu-ray/DVD release of the third "Girls und Panzer das Finale" film.
One day at Oarai Girls High School, the five members of the Anglerfish Team are looking for a student named Jane, who belongs to the Agriculture Department, to deliver some documents of the Student Council. After unfamiliar horse riding, they finally arrive at the Agriculture Department building, where they meet Jane, who seems to be ready for battle with a gun in her hand. According to her, she is in the middle of chasing the "Arch Villain of the Century"...!
(Source: Crunchyroll)
— Description from AniList community (anilist.co).
