Tokiro Ehara, an idealistic geeky professor, begins teaching at an exclusive high school for girls, with hopes of meeting young innocent minds in their most fragile state. Unfortunately, on his way to work, he meets 18-year-old Miyako and finds out that she is one wild student that needs to be taught how a prim and proper teenager should behave. He starts to believe that he is just the right person to teach her. Miyako, however, has other plans, and she's not above using her seductive wiles to lure him into submission.

After fending off the threat of a Neuroi invasion of Romagna and destroying the enemy's nest over Venezia, Yoshika Miyafuji goes back to her home town in the Empire of Fusou. Despite the loss of her magical and healing abilities, the former officer of the 501st Joint Fighter Wing wants to continue studying medicine. This is in order to help those in need, both civilians and those on the front lines alike. She receives an invitation from a prestigious school in Europe and decides to accept the offer, embarking on a journey back to the war-torn continent. However, a new danger arises, as more and more Neuroi manage to break through human defense lines, appearing unnoticed in supposedly safe areas and disturbing the balance of power in Europe. Yoshiki's sense of duty collides with her inability to pilot a Striker Unit, but with her friends in peril, it seems that there is only one thing she can do.

On the far side of the Milky Way, there is a planet gestating magical power-Wilhallen. There are four very different countries on this magical continent: the ancient and mysterious eastern empire—Huang; the free and open Nanyang island country—Tapaz; the iron-clad western republic—Vrsaris; and, Civilized and elegant Beilian Gang-Agnesa. All countries enshrine the High God and obtain the gift of divine power. Although there are occasional conflicts, most people enjoy a peaceful life. Until the era of 3836, when a mysterious man in black suddenly came with an unknown dark force, but found himself in the wrong place at the wrong time. Fortunately, the established fate was as recorded in the time document in his hand. Verhallen opened a new parallel world... (Source: bilibili, translated)

Sayonara Zetsubou Sensei OVA.