This is the inside of a human body.
One day, as Red Blood Cell is making her routine oxygen and carbon dioxide deliveries, she becomes the target of Pneumococcus Bacterium that suddenly invades the body. She is saved by White Blood Cell (Neutrophil), who then goes after Pneumococcus on the run. His job is to protect the body that is their world.
But the enemy finds a hideout in an unexpected place…