BACKGROUND: Increasing presentations to the Emergency Department (ED) via police (which include detainees, prisoners and community residing persons) and the increase in chronic and mental health illness in detainee and prisoner populations has prompted an increased requirement for healthcare delivery within the custodial environment. This study aimed to describe the Watch House Emergency Nurse (WHEN) role, focusing on structures and processes underpinning the role. METHODS: In this qualitative, descriptive study, semi-structured interviews were undertaken in 2015 with 14 key stakeholders from health, police, and ambulance services. Interviews were analysed using content analysis to inform the findings. FINDINGS: Important structural elements of the WHEN role included an ED triage competent registered nurse, a 2-day integrated training program, and clear guidelines to provide a framework for identifying, prioritising and managing healthcare needs. Important process elements were clear communication between nurses, police, and medical staff, and a clear understanding of roles and responsibilities to facilitate continuity of care and appropriate referral. The underpinning perceived benefit of the WHEN role was 'safety'. This was in terms of personal, professional, and detainee safety. CONCLUSION: The structures and processes underpinning the innovative WHEN role provides a valuable foundation for guiding evaluations of other nursing roles in other early custody settings.