From Mangahelpers:
After two years of being dormant inside Tsutomu Senkawa, an accidental encounter with their old nemesis violently wakes Birdy up. With Birdy kicking evil's ass once again, the dark secrets of the experiments conducted by the "Agni" corporation are slowly revealed. They seem to be dabbling with human body enhancement, creating what they call "beast men," to combat the alien invasions related to Birdy.