"Flu Before Boo" follows a brother and sister who are very excited to be superheroes for Halloween. However, they are not nearly as excited about having to get their flu shot. They soon learn all about the team of superheroes hard at work within their bodies: The Immunity Crew! These heroes practice their battle skills on the practice villain (the vaccine) so that when the real flu comes along, they are ready to fight and destroy it before it can cause any harm. After learning how their cells are heroes just like them, brother and sister Louis and Jenna are proud to do their part in the battle against viruses.