Third season of Ai Mai Mii.

Manabi Straight! follows the lives of a group of young high school girls living in the year 2035 while they attend the all-girl Seioh Private High School. Since the birth rate has dropped dramatically, schools are being closed down due to the sheer lack of students available to teach. Morale in schools has dropped dramatically, and Seioh is no exception. The story begins when the main character, Manami Amamiya, transfers to Seioh. After some initial hijinks involving a futuristic scooter and a swim meet, followed by an inspirational school song performance, she is inducted as the student council president. The story that follows pertains to Manami working with Mika Inamori, the only other student council member, and three other classmates named Mutsuki Uehara, Mei Etoh, and Momoha Odori, in student council matters. After some remodeling of the student council room, Manami and her friends set forth to plan for the upcoming student festival. (Source: Wikipedia, edited)

The story took place in the early days of the Sirius War. Newcomers on the battlefield fight bravely. Proving their value as a new generation of super-warriors on the front lines. However, when they followed their superiors, they were attacked by the Black Army, and the chiefs died. The battlefield fell and was forced to retreat. Comrades in the previous battle were just ordinary humans. They think that they died in the attack of the black army, but it turns out he survived.

Thirty-two years have passed since the initial defeat of the Invaders. A.E.G.I.S is all but dismantled now and has become an underground organization consisting of only a few Gate Keepers left to save humanity. One of the members, Ayane Isuzu, is sick of society as a whole, though she continues to defeat the Invaders out of spite for her father. Suddenly, she learns of the revival of Machine General (Kikai Shogun) and Devil Count (Akuma Hakushaku), but a mysterious ghost girl now accompanies them. It is up to Ayane and the remnants of A.E.G.I.S to stop them and save humanity once again. (Source: ANN)