A naive and idealistic rookie red blood cell named AA2153 starts his first day of work and is overwhelmed by the state of the body he works in. The blood vessels are clogged by cholesterol, the red blood cells are not allowed to take breaks, and everybody is grumpy and rude due to the body being under stress. Delivering oxygen with a veteran red blood cell, they face a carbon monoxide cloud and are saved from a group of Pneumococcus bacteria by a white blood cell named U-1196...