This is a story that happens inside a human body. One day, as a Red Blood Cell is delivering oxygen and carbon dioxide, she is assaulted by Pneumococci and rescued by a White Blood Cell. To protect the body that’s their world, the White Blood Cell (Neutrophil) chases after the one Pneumococcus who’s escaped. But the enemy has hidden himself in the most unlikely place...

Things have been peaceful for Red Blood Cell when suddenly an abrasion occurs before her eyes. The White Blood Cells (Neutrophils) arrive to battle the bacteria and viruses invading through the wound, but they struggle against enemies like Staphylococcus aureus. Will the White Blood Cells (Neutrophils) find a way to defend this body?

Naive T Cell is patrolling the body for influenza virus proliferation. But since he’s never taken on the enemy before, he’s completely terrified and useless. He finally ends up fleeing the battlefield where White Blood Cell (Neutrophil) and the veteran Killer T Cells are fighting. Naive T Cell is filled with self-loathing until Dendritic Cell sees him and speaks to him kindly.

Bacteria appear near the stomach. White Blood Cell (Neutrophil) is on the scene at once. There, he finds Eosinophil, another type of White Blood Cell, already in battle, but she’s struggling badly. Seeing how powerless she is, the other cells start trashing Eosinophil behind her back. Even after White Blood Cell (Neutrophil) tells them, “There’s a job that only Eosinophil can do...”

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.

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?

A mysterious cell wearing a stylish hat appears before the regular cells, who are bored with just the same (cell division) work repeatedly, every day. This mysterious cell lures the regular cells into a mischievous scheme against White Blood Cell and Killer T Cell. The regular cells enjoy working out their everyday, pent-up frustration. But just who exactly is this cell wearing a stylish hat?

Memory Cell sees a dream where the arrival of messengers from the outside signal the end of this world... As if his dream became reality, unidentified enemies enter the body! Does Memory Cell not only have the power to remember the past, but also predict the future now? Will this world be ruined as Memory Cell predicts?

Ordinary Cell heads to the intestines with White Blood Cell (Neutrophil) to reunite the lactic acid bacteria with their friends, but they get separated from one of the bacteria on the way there. Ordinary Cell desperately searches for the lactic acid bacterium but is attacked by cells infected by a new virus!