Mental health day treatment (MHDT) programs provide intensive group psychotherapy for patients with psychiatric pathology complicated by personality disorder. Recently, researchers have begun to examine specific components of these programs. Of importance is the theoretical rationale, which may be challenging to understand given the complexity of the treatment. The purpose of this project was to investigate the theory of one MHDT program. Community-based participatory research was chosen and accordingly, all stages of the project were collaborative with the MHDT clinical team. We engaged in a six-month, iterative process of weekly action-reflection cycles wherein material was discussed, analyzed for themes, and the findings presented back to the team to further the conversation. Results summarize this program’s Theories of Dysfunction and Therapeutic Change, which were primarily psychodynamic, but also integrative through assimilation of elements from other paradigms. Usefulness of the research process is discussed and recommendations are provided for others wishing to undergo a similar process.