This article analyses the production and reception of the natural history film series Secrets of Nature (1919-33) and its sequel Secrets of Life (1934-47), exploring what these films reveal about the role of cinema in public discourses about science and nature in interwar Britain. The first part of the article introduces the Secrets using an 'intermedial' approach, linking the kinds of natural history that they displayed to contemporary trends in interwar popular science, from print publications to zoos. It examines how scientific knowledge was communicated in the series, especially the appeal to everyday experience as a vehicle to engage mass audiences with scientific subjects. The second part examines the Secrets series through the lens of knowledge co-production, detailing how a range of different figures, including academic scientists, nature photographers, producers and teachers, became entangled in making the films. Recovering the term 'ciné-biology', it argues that Secrets developed a unique style of filmmaking that generated cultural space for the life sciences in British popular culture. The third part analyses two interwar cinema experiments to explore how audiences, imagined and real, shaped the kinds of natural knowledge characterized by the Secrets films.