Bert the circus mouse is missing! The members of Sir Sidney’s Circus look everywhere. They don’t know that tiny Bert has been carried away by a huge balloon. Meanwhile, far away, Flora Endora Eliza LaBuena LaPasta is tired of being ignored. Her parents have no time for her as they prepare for a new baby, so Flora comes up with a plan to mail herself to her friends at the circus. A popped balloon and a mixed-up shipment bring Bert and Flora together. In the worst of times, a best friendship is born. But can a circus mouse and a runaway girl make it through a world of bad drivers, angry mobs, and greedy robbers? The pals must use their wits to stay out of trouble and try to find their way home in Pop Goes the Circus!