Tetsurou Okino, a young engineer, is transferred to Hato Industries, a vermin extermination company, bringing with him Bullbuster, a robot he developed himself. Hato Industries is a small, cash-strapped firm. Its president, Tajima, faces off against a mysterious creature. Fuel costs for robots and labor costs for pilots are high, and wasting even a single bullet is unacceptable. Caught between their extermination ideals and budget realities, what lies ahead for Hato?