Let me become a great red blood cell one day... This is the story of Red Blood Cell before she became Red Blood Cell, when she was a young Erythroblast. The Erythroblasts are training hard to become full-fledged Red Blood Cells under the supervision of their teacher, Macrophage. But Erythroblast is directionally-challenged, and she’s always getting lost in the middle of drills. One day, while practicing to evacuate from bacteria, Erythroblast once again loses her way...

Delivering carbon dioxide to the lungs, and bringing oxygen to cells throughout the body... That’s Red Blood Cell’s important task. Though she’s still directionally-challenged and always losing her way, she can’t be a fledgling forever. She’s determined to go through Circulation by herself without any help from the older cells, but in the shadows, White Blood Cell (Neutrophil) watches over her with a worried look on his face.

In the middle of a drill, Killer T Cell tosses a subordinate right into Helper T Cell’s office. The two polar opposites start arguing. According to Dendritic Cell, although they play very different roles today, they were actually classmates at Thymus School. Not only that, but they were totally different back then! What was it that developed between these two during their days of enduring hellish training sessions at Thymus School where only two to three percent of cells manage to survive?

Red Blood Cell has been assigned to train a rookie Red Blood Cell! She resolves to “show her what a dependable senpai I am!” But she keeps losing her way, and getting yelled at by other cells, so it’s not going very smoothly. Just then, a major incident rocks the world inside the body.