Johnny Redblood and his best friend Ashley are two of the fun loving inhabitants of Micropolis - a city located within a cheerful ten-year-old girl named Sarah. Besides them, Micropolis is also filled with different types of single-celled creatures that live and work together and play a vital role in what Sarah does and how she feels.
As a red blood cell, Johnny Redblood has a very important job. He works for the Oxygen Delivery Service, bringing the valuable resource to the residents of the city. But his daily routine abruptly changes one day when Sarah steps on a spiky sea urchin, and Tyrus the Virus enters the lower valley. As the virus begins to cause serious damage, Johnny and his friends must find a way to stop Tyrus and save their beloved city!