Project A-Ko 4: Final (movie, 1989)

A-ko and B-ko are fighting as usual, as C-ko watches from the sidelines feeling neglected by both. Ayumi-sensei has been behaving strangely lately. She's wearing a heart-shaped crystal with a hexagram inside, and is chanting mysterious prayers in private. At the ruins of an ancient civilization, an archaeologist spies a hexagram among the hieroglyphics. Deep in outer space, an alien fleet is on a course for Earth, and its flagship bears the same hexagram shape.