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