Inside a human body, a red blood cell, named AE3803, is rescued from a sudden attack from a group of Pneumococcus bacteria by a white blood cell, named U-1146. Later, as AE3803 tries to find her way to the lungs to make the delivery, she discovers one of the bacteria has escaped and is planning an attack on the lungs, so she and U-1146 accompany each other to the lungs to pursue it. Upon reaching the lungs and parting ways with U-1146, AE3803 discovers Pneumococcus had been hiding in her package the entire time, waiting to target red blood cells for their nutrients. However, U-1146, who had caught onto the bacterium's plan, arrives in time and lures the bacterium into a trap, where he is ultimately sneezed out of the lungs.