The cluster randomised controlled trial found that adolescents who participated in the ACDC programme experienced greater reductions in depressive symptoms compared with those receiving usual care. The intervention was also associated with significant improvements in cognitive processes linked to depression, including reductions in rumination and perfectionistic attitudes. Designed as a low-threshold group intervention, the programme combines cognitive behavioural techniques with elements from metacognitive therapy, positive psychology, and neurobiological approaches to mental health. Overall, the findings support the effectiveness of group-based CBT as an accessible intervention for reducing depressive symptoms and associated cognitive vulnerabilities in adolescents.