The body has been invaded by a mass of cedar pollen! Rushing to the scene so that he can eliminate the cedar pollen, White Blood Cell (Neutrophil) runs into Memory Cell. Memory Cell is a Lymphocyte who retains the immune system’s memories. Seeing what’s actually going on, Memory Cell is strangely flustered. “What the heck? This is just like the legend...” What “legend” is Memory Cell talking about?

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...

The enemy disguising himself as a common cell has revealed his true identity. He is Cancer Cell. To protect this world, White Blood Cell (Neutrophil), Killer T Cell, and NK Cell must now confront the attacking Cancer Cell in battle.

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 is being chased by a germ! The one to come to the rescue is a masked Monocyte. Apparently, they are also part of the White Blood Cell family and a type of Immune Cell. There’s no way Red Blood Cell could be attacked by bacteria twice in one day, but she encounters another one on her way to the nasal cavity. The germ turns out to be Staphylococcus Aureus. They usually just exist on the skin and pores. But this time, they have a secret plan. What could it be?

The world (body) is engulfed in sweltering heat. Even after triggering sweat, the body can't be cooled down. Abnormal increased vascular flow, soaring fever... That's right, the body is suffering from heat stroke! Taking advantage of the situation, the Bacillus cereus is rampaging to his heart's content. Reeling from the effects of heat stroke, the usually reliable White Blood Cell (Neutrophil) is suddenly unable to perform... Isn't there any way to save the world from further global warming?

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.

"We have detected life-threatening damage!" Hearing of this emergency, White Blood Cell (Neutrophil) rushes to the blast area. But there, he finds no traces of the blood cells he's supposed to protect! Due to massive bleeding, a huge amount of blood cells have already been flung outside the wound. And the bleeding continues... The situation grows more dire every second, but to protect this world and to save her friends, Red Blood Cell continues to deliver oxygen with Li'l Kouhai Red Blood Cell. Their world is on the brink of destruction... What kind of fate lies ahead for the cells?