When Hector returns to school his classmates want to know what made him so sick that he missed the soccer game. Ms. Thomas has prepared a very special lesson to answer their questions, and AC helps with time travel magic. Hector wants to know what germs are, so AC introduces the class to "Virus" and "Bacteria," then "beams" the class in and out of places like the gym and playground where the kids find out how germs can get into a person's body. When they return, the kids ask why we are not sick all the time, if germs are all around and even in our bodies. Ms. Thomas tells them about the body's natural defenders, the "fighter cells." But kids do get sick sometimes and when that happens, they may have to go see a doctor and take medicine to help the white fighter cells beat the germs. AC time travels to a pharmacy to show them why they must take medicines according to the doctor's instructions. Finally, the class learns how to stay healthy and keep their "fighter cells" strong by eating the right foods, getting lots of exercise and a good night's sleep, and how to keep germs out of their bodies by staying clean.