I think that he was sent to the moon to protect the Talisman there somewhere between the days of king Arthur. Once he reach the moon he went to stasis lock to not waste energon. When the events of dark of the moon happened he was reactivated because of the energon from the pillers and unknowingly join the Decepticons but when he found out that the Decepticons were attacking humans he attacked them using his ship but was quickly outnumbered and was shot down in Chicago
