In this paper we explore the enfolding spatialities of control and care within the penal estate through analysis of the creation of a unit for prisoners with serious mental illnesses (SMIs). Prisons have increasingly become the key institutions for mental health care provision, yet serious mental illness disrupts the self-government upon which contemporary prison regimes are based. Our analysis highlights the ‘trouble’ institutions face in making space for mental health care; in trying to fit different control-care regimes into existing carceral environments. We argue that the different actors that have made space for this control-care have been open to potentiality. Developments have been experimental, emergent and incomplete; often not officially challenging wider institutional processes, but eluding them. What emerges is an institution within an institution with a certain level of spatial autonomy but constrained in its transformative potential.