Inside the human body, there are approximately thirty-seven billion cells that diligently perform their duties every day. New to her position, Sekkekkyuu AE3803 is clumsy but cheerful; she is excited to begin delivering oxygen and collecting carbon dioxide, despite her poor sense of direction. Meanwhile, Hakkekkyuu U-1146 is always prepared to fight and kill any pathogen he encounters. After running into each other multiple times, they quickly become good friends, regardless of their contrasting personalities. Entertaining and educational, Hataraku Saibou follows the two blood cells and their many comrades as they work together to keep the body healthy.