Every year, 100 million Monarch butterflies set off on an incredible journey across North America. These beautiful creatures fly 2,000 miles to reach their remote destination: a tiny area high in the mountains of Mexico. Yet scientists are still puzzling over how the butterflies achieve this tremendous feat of endurance — and how, year after year, the Monarchs navigate with such hair’s-breadth precision. NOVA flies along with the Monarchs, visiting the spectacular locations they call home and meeting the dangers they encounter along the way. As this program reveals, the Monarch is a scientific marvel locked in an inspiring struggle for survival.