In recent decades, the discipline of International Relations (IR) has experienced both dramatic institutional growth and unprecedented intellectual enrichment. And yet, unlike neighbouring disciplines such as Geography, Sociology, History and Comparative Literature, it has still not generated any ‘big ideas’ that have impacted across the human sciences. Why is this? And what can be done about it? This article provides an answer in three steps. First, it traces the problem to IR’s enduring definition as a subfield of Political Science. Second, it argues that IR should be re-grounded in its own disciplinary problematique: the consequences of (societal) multiplicity. And finally, it shows how this re-grounding unlocks the transdisciplinary potential of IR. Specifically, ‘uneven and combined development’ provides an example of an IR ‘big idea’ that could travel to other disciplines: for by operationalizing the consequences of multiplicity, it reveals the causal and constitutive significance of ‘the international’ for the social world as a whole.