This paper reported the therapeutic project from a serious mental disorder with heavy obsessive-compulsive psychopathology in a young adult. The treatment was possible with the intense involvement of the father. The paradigm of reference is the Multipersonal Psychotherapy which requires a long psychotherapeutic work in mobile setting with the patient alone, with his father alone, with father and patient together with two professionals who guarantee the groupal and family format of the psychotherapeutic field. The goal is to think of his father as "person", the bearer of a life story that makes meaningful its existence, in fact, as a person and as an organizer of mental life of children. So the focus was not on the paternal functions or roles, but on the elaboration, together with the son, of the tragic drama that captured his life, their lives. Psychotherapy has allowed mental co-evolution that has healed widely the psychopathology and mobilized the hope and competence to new paths of existence.
Keywords: Obsessive-compulsive disorder, multipersonal psychotherapy, mobile settings, father as a person and not as a role, death, co-evolution of parent-child.