Caterpillars open the film as a test case: long assumed deaf, they turn out to respond to spoken vibrations by thrashing their poisonous hairs, the same defense they use against predators. Moths, the adult form of those caterpillars, show the same acoustic sense at work against bats, dropping out of the air the instant they detect a hunting bat's whistle. The film moves underwater to dolphins and whales, which send low-frequency calls through water over distances up to 500 miles, and compares that to the shape of external ears on land animals and how those shapes sharpen or blur what an animal hears. A flock of cranes provides the aerial case study, their constant calling serving as both contact signal and migration compass, a system the film likens to the acoustic coordination used between aircraft fleets. Cicadas, tarantulas, toads, and birds each get a shorter segment, and the film closes by turning the same questions on tribal human communication, treating sound as a survival tool shared across species rather than a uniquely human trait.