Orange-and-black wings fill the sky as NOVA charts one of nature's most remarkable phenomena: the epic migration of monarch butterflies across North America. To capture a butterfly's point of view, NOVA’s filmmakers used a helicopter, ultralight, and hot-air balloon for aerial views along the transcontinental route. This wondrous annual migration, which scientists are just beginning to fathom, is an endangered phenomenon that could dwindle to insignificance if the giant firs that the butterflies cling to during the winter disappear.

• Documentary