While there is a burgeoning literature critically mapping the spatial logics of immigration detention around the world, there is relatively little systematic research on geographies of resistance, particularly the role of ‘outsiders’ – members of the public with relatively secure status. This article considers how people mobilise around immigration detention in the UK to challenge the status quo. Drawing on qualitative interviews and survey research, it offers in a fine-grained analysis of what nourishes civic mobilization, considering the concerns and positionalities of volunteers. It examines their experiences of taking action, visiting detention centres or campaigning for change, highlighting rewards and challenges. Probing divergences as well as convergences in people’s approaches to the issue, a picture is built up of a vibrant detention movement working across multiple spaces and scales against government efforts to isolate, contain and exclude ‘unwanted’ migrants. How this civic mobilization challenges moral distance and political closure offers a fresh insight to the geography of detention and study of pro-migrant mobilisation.