In a Philadelphia garden built by two boys to bring butterflies, a tiny egg laid on a skinny milkweed leaf shakes and hatches. Mona monarch, a butterfly superhero, is born. Her task is to fly thousands of miles from her summer home in Philadelphia to her winter home in the mountains of Mexico. Noah and Nate follow Mona to Mexico, but they must take cars and planes, buses and horses, to reach the place where Mona has flown alone. There they learn that these amazing monarchs are in danger of disappearing. So, they make a promise to Mona and her monarch friends to do everything they can to save them from extinction.