Unpredictable and complex developments challenge the application of strategic environmental assessment (SEA), for example, in terms of timing, prediction and relevance of assessments. In particular, multi-actor and unstructured strategic-level decision-making processes often seem to be characterised by unpredictable and complex changes. Despite the apparent implications, explorative investigations about how unpredictability influences SEA application in practice are rare. This article aims to shed light on contextual changes and reactions to such changes in practice by a case study of the specific SEA process of the Danish Natural Gas Security of Supply Plan. Special emphasis is given to the framing of alternatives in the SEA process, since alternatives are directly related to the contextual developments. Based on a participative approach, strategic dynamics are mapped and the reactions and concerns in the SEA team are outlined. The contribution of the article is enhanced empirical understanding of strategic dynamics and how it challenges SEA application.