During the heyday of the studio system in Japan in the 1950s, Mizuki Yōko (1910–2003) was one of Japan’s most prominent and celebrated screenwriters. Despite screenwriting being a markedly homosocial profession, Mizuki forged a remarkable career as a freelance writer, working both for major studios and independent productions. Her collaboration with directors such as Naruse Mikio and, above all, Imai Tadashi resulted in a string of critically acclaimed films. While Imai’s films were lauded by contemporary critics, his approach to directing has subsequently been regarded, especially by western scholars, as somewhat impersonal and his sympathies too leftist. Conversely, these social issue (shakaiha) films, often based on original screenplays by Mizuki, scrupulously displayed the anxieties and ambiguities of the post-war era when the social fabric of Japan was radically reconfigured as its people embraced the newly imported values of democracy and consumerism. In this article, I examine the contributions of Mizuki to the oft-neglected oeuvre of Imai and social issue film in particular. I argue that besides pointing at the capacity and bounds of narrative cinema to engage with timely and sensitive social topics, Mizuki’s working methods underline a screenwriter’s awareness of her own agency in filmmaking.