Metal Skin Panic MADOX-01

The Army's new combat unit, the MADOX-01, is lost in transit after the convoy in which it was being transported crashed. It ends up in the hands of a mechanic by the name of Kōji. It doesn't take him long to get himself into it, but then he finds out that it won't let him out. All he wants to do is see his girlfriend, but a division of tanks is out to retrieve the MADOX - and it's set to defend itself.

Synopsis from Anime News Network