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...

"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?