The practice of teaching involves constant learning and professional growth. Professional development (PD) is necessary even for qualified in-service teachers, allowing them to enhance their disciplinary knowledge, acquire and become familiar with new pedagogical approaches, and deepen their content-pedagogical skills. Educators have suggested incorporating action research (AR) -- in which teachers conduct small-scale research in their own classes -- into PD of K-12 in-service teachers. An effective combination of PD and AR should, among other things, be long-term, be connected to the teachers' practice, and rely on the perspective of teachers' pedagogical content knowledge (PCK). We report on a unique PD on AR for in-service computer science (CS) K-12 teachers. This PD included a longitudinal workshop for experienced high-school CS teachers. The teachers performed AR in their own classrooms, investigating topics that they chose, based on their own personal practice, and motivated by authentic teaching challenges that they faced. In line with situated learning theory, the workshop was conducted in a rich social context. This context allowed for constant constructive feedback from peers and fostered effective reflection processes. Our findings indicate that the unique experience that the teachers underwent within the social context significantly influenced and improved their practice.