Inside a human's body, roughly 37.2 trillion cells work hard everyday endlessly to keep their host healthy. AE3803, a new, ditzy red blood cell was attacked by a group of pneumococcus bacteria during the midst of her job. U-1146, a neutrophil appears and ruthlessly terminates the bacteria before they could do any harm. AE3803 thanks U-1146 for saving her and they part ways.
As AE3803 wanders around looking for the lungs, she decides to check the map and enters the map room. However, upon opening the door, she finds Pneumonia Coccus, who is also looking at the map. The two are surprised and AE3803 closes the door, only for Pneumonia Coccus to smash the wall down and yell at her for pretending as if nothing happened. U-1146 finds the two and battles Pneumonia Coccus, but the bacteria escapes by tossing a bacterial shell to create a smokescreen and pin U-1146 down.
U-1146 offers to take AE3803 to the lungs, which she accepts. With the threat of the lungs being infected, Helper T Cell authorizes the Killer T Division to hunt down the bacteria. The division mocks AE3803 and U-1146 tells her not to pay any attention to them. They cut their way through the Dendritis Tree to take shortcut to the lungs. However, they find the tunnel closed and a Platelet informs them that because of unforseen circumstances, no one can pass. The two cells notice that the platelets are struggling to open the boxes as they are too short, so they assist them and are allowed to go through. On the way, AE3803 notices U-1146's receptor is acting strangely, and he states that it goes off whenever a virus or bacteria is nearby.
They arrive at the lungs where the two cells part ways. U-1446 notes that his receptor has stopped shaking violently, and realizes that something is off. Pneumonia Coccus bursts out of the box AE3803 was transporting, and thanks her for giving him a free ride to the lungs. He causes havoc, but U-1146 quickly intervenes and states that he realized that he hiding in the box after he left AE3803. U-1146 is unable to kill Coccus because of his bacterial shell, so he lures him into the respiratory tract. Not realizing what he is up to, Coccus steps onto the red line and triggers an alarm; U-1146 presses a button, and the bacteria is trapped in a shell. The two cells take some tea and watch as the shell is deposited into a missile and ejected from the body via sneezing before separating ways.
Around the end of the episode, when the captions read "Incident! It's Bacteria! They're close!", the receptor from U-1146's hat disappears for a frame/split-second.
For a moment after the main platelet apologizes, the team's sign is shown to be behind the striped caution sign. But in the next shot, the platelet sign is in front of the caution sign, and closer to the main platelet.