The story begins with Aoba Watase, a boy who goes to high school in Tokyo and lives a carefree daily life. On the morning of his school’s opening ceremony after summer vacation, he goes to campus as usual, but he has a chance encounter with a girl named Hina Yumihara. Their encounter will soon alter the fate of the world. (Source: wikia.net)
Buddy complex is a mecha anime that also depicts time travel. The tv series showed how Aoba was traveled into the 70 years in the future and meet someone he knew, Hina, who is on the side of the enemy.
Unknown on both parties, but it was them, Aoba and Hina, who made themselves travel in the future. In the OVA, it was explained how they traveled.
Though I prefer the OVA to be a much longer series, like making it a second season, but they explained everything in the OVA and also how Bizon was caught in it. If you feel confused, try watching it once again and note how everything happened from the very beginning.