Mental health problems are a serious set of issues for youth with neurodevelopmental disorders, a term that refers to children diagnosed with autism spectrum disorder, fetal alcohol spectrum disorder, cerebral palsy, learning disabilities or attention deficit hyperactivity disorder (ADHD). These conditions are estimated to affect 16% of children. Between 40-70% of children with NDD have serious mental health problems, like anxiety or depression, and evidence-based treatments are needed. The current project will test whether a parent-involved cognitive behavioural intervention (Secret Agent Society: Operation Regulation; SAS-OR) improves the emotion regulation abilities and decreases emotional and behavioural problems in children with NDD. SAS-OR is delivered as individual therapy, with the content of each session framed within an engaging theme of the child being a secret agent in training, learning to decode and handle emotions for their missions. We will involve 160 children, 8 to 12 years of age, who have an NDD and a mental health problem. Half of the children will receive SAS-OR immediately while the other half will need to wait to receive it, and we will measure changes in emotion regulation and mental health in both groups. This research stands to result in major system change, enabling the delivery of evidence-based treatments for mental health problems in youth who are often excluded from receiving mental health care as a result of their complex sociocommunicative and cognitive profiles, yet who are the most vulnerable to mental health problems to begin with.