This paper illustrates the relevance of group therapy using theatre mediation based on the case of Corentin (14 years old) and its psychoanalytic analysis. In spite of a longstanding psychotherapy in a Centre Médico-Psycho-Pédagogique (CMPP) (center for children and youths aged 0 to 18 years old suffering difficulties in their development, sponsored by the French Ministry of Social Affairs), therapists are preoccupied with Corentin and fear a psychotic crystallization of his personality. He feels persecuted by all, especially by his father, his therapy gets limited results, and his social relationship with his peers worsens. He was offered group therapy using theatre mediation as a way of giving new impetus to his therapy. Through games of improvisations, Corentin experiments identification with the father and commits his imaginary murder. Progressively, he discovers the symbolic dimension of this murder thanks to the organization of the group and its elaboration capacity. This young teenager gets involved anew in his psychotherapy, goes back to reality and creates new social links with his peers.