Mary Shelley mined the ideas of international thought to help develop three new subgenres of modern political science fiction (‘poliscifi’): post-apocalyptic, existential, and dystopian. Her two great works of poliscifi, Frankenstein (1818), and The Last Man (1826) – confront the social problems that arise from humanity's technological and cultural interventions in the wider environment. This article recovers The Last Man not only as the first modern post-apocalyptic pandemic novel, but also as an important source for the existentialist tradition, dystopian literature, and their intersections with what I call ‘Literary IR’. Comparing The Last Man with its probable sources and influences – from Thucydides and Vattel to Orwell and Camus – reveals Shelley's ethical and political concerns with the overlapping problems of interpersonal and international conflict. The Last Man dramatises how interpersonal conflict, if left unchecked, can spiral into the wider sociopolitical injustices of violence, war, and other human-made disasters such as species extinction, pandemics, and more metaphorical ‘existential’ plagues like loneliness and despair. Despite these dark themes and legacies, Shelley's authorship of the great plague novel of the nineteenth century also inspired a truly hopeful post-apocalyptic existential response to crisis and conflict in feminist poliscifi by Margaret Atwood, Octavia Butler, and Emily St. John Mandel.