On the first day back after summer break, high-school student Aoba is attacked by a giant robot that appears out of the sky. As he's pursued through the city, his classmate Hina appears in a robot of her own. She rescues him, and tells him cryptically that "Dio is waiting for you," before she disappears. This begins Aoba's new life as the pilot of the Free Treaty Alliance against the Great Zogiria Republic.
I'm not a real fan of mech animes.. But this one seems pretty cool.