Cancer rehabilitation in Sweden is fragmented and unequal, leaving many patients and their families without adequate support after clinical treatment ends. This ongoing study evaluates Kraftens hus (KH), a patient-led social innovation designed to complement traditional cancer care by offering emotional, social, and practical support through civil society engagement. Using a modified realist evaluation framework, we investigate how KH functions across micro, meso, and macro levels, and under what contextual conditions its mechanisms lead to beneficial outcomes. Data collection includes qualitative interviews, surveys, document analysis, and focus groups across four KH sites (Borås, Göteborg, Stockholm/Gotland, Östersund). At the micro level, we explore how KH supports individuals in navigating fragmented care systems and enhances their sense of coherence and self-efficacy. At the meso level, we examine the organizational model of KH and its capacity to foster cross-sectoral collaboration. At the macro level, we assess how patient involvement and co-production contribute to system-level change and sustainability. Our initial program theory suggests that KH improves quality of life for cancer-affected individuals by activating mechanisms of peer support, empowerment, and resource integration. The organizational model is expected to facilitate trust and legitimacy among stakeholders, while patient-driven processes enhance relevance and responsiveness. This empirical study contributes to the growing field of social innovation in healthcare by demonstrating how civil society plays a complementary role in rehabilitation. It will also offer a transferable model for other patient groups and health systems seeking to address complex needs through inclusive and sustainable approaches.