Complex sentence: The story starts with an amnestic Tenrei Gundam waking up as he walks trying to recall his past he was attacked by the Hyakki Yakou Shuu who claim that he is a Gundam.
Split into simpler sentences: The story starts with an amnestic Tenrei Gundam waking up. As he walk, trying to recall his past he was attacked by the Hyakki Yakou Shuu who claim that he is a Gundam.