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.