The series "Once Upon a Time... Life" used the same recurring lead characters as the other "Once Upon a Time..." series: certain represent the cells that make up the body's systems and defense mechanisms, such as red blood cells, white blood cells and platelets, while antagonists represent viruses and bacteria that threaten to attack the human body. Every episode of the series featured a different organ or system within the human body (like the brain, the heart, the circulatory system, etc.).