Microbiological contamination of drinking water supplies is an ever-present concern for water utility managers. Most such threats are routine, well-recognised and described. Therefore, they can usually be prevented using standard protection measures. Incidents involving emerging pathogens and malicious attacks are inherently less predictable. In a multi-stage process over one day, participants with backgrounds in microbiology, medicine, infrastructure, data analysis, environmental or public health and facility management developed qualitative scenarios on potential threats posed by either an emergent pathogen in or a microbiological attack on drinking water supplies in a European country. Participants were guided via structured activities to identify key factors that would impact the magnitude and severity of such an emergency. Plausible variant states for each key factor were determined, and participants constructed sequences of events to create scenario outlines. Five scenarios in outline form are reported which incorporate genuine possible future events as well as pathogens of international concern. Common features that would exacerbate all scenarios were under-investment in public services, inadequate water quality testing, and monitoring and lack of resources to keep water supplies safe. Participant evaluation of their scenario planning experience was broadly very positive and the scenario planning process was received as credible and relevant.