A growing number of inmates currently incarcerated in France suffer from a psychotic disorder such as schizophrenia. The purpose of psychiatrists working in prison is to give them care. Within the prison of Dijon, we developed a therapeutic group for our patients with such disorders, and with residual symptoms despite effective treatment. Michael's Game, a therapeutic tool conceptualized by Y. Kazhaal and J. Favrod, is a game designed to work on these cognitive symptoms, through a playful training to assumptions’ reflection. This work presents the development of this group on a small sample of patients in prison, the peculiarities of working in detention and therapeutic effects obtained. These effects are measured by two scales: the Beck Cognitive Insight Scale (BCIS) and the Peters Delusion Inventory-21 items (PDI-21), administered to participants one week before the beginning of the group and one week after its end.