Montezuma Castle, a five-story 20-room cliff dwelling, served as a " high-rise apartment building" for prehistoric Sinagua Indians over 600 years ago. Now it serves the same purpose to a colony of bats. Concern over the possible harm that these latter-day residents may be doing has led to some very interesting hands-on research to study the impact of bat habitats on cultural landmarks. The program also visits Tonto National Monument, where the scale of the ruins makes capturing bats a test of wills and ingenuity, and Chiricahua National Monument to see how one of Arizona's natural gems is the perfect location for collecting data on tree-roosting bats. This episode finds bats in hand, in roosts and in the air to learn how bat population density affects humans.