NOVA follows a number of scientists, including Matthew Wilson of MIT, who is literally "eavesdropping" on the dreams of rats, and other investigators who are systematically analyzing the content of thousands of human dreams. From people who violently act out their dreams to those who can't stop their nightmares, from sleepwalking cats to the rare instances of individuals who don't seem to ever dream, each fascinating case study contains a vital clue to the age-old question: What Are Dreams?