It's a dangerous, germ-filled world out there, and Little knows it. Little is a young white blood cell with a mission to protect The Human from any pathogens that try to cause infections. However, Little isn't as strong as the other white blood cells, and he's certainly not brave, either. Even his sister Tiny is not afraid of anything. They, along with their new friend Mike, soon discover a new virus that is trying to weaken The Human's immune system! Will Little, Tiny, and Mike be able to stop this virus before the immune system collapses?