Experiencing sexual coercion during adolescence can lead to adverse psychological and physical health outcomes for those affected. Eliminating such experiences is important for enhancing adolescent wellbeing, and the provision of good quality relationships and sex education (RSE) is needed. Engaging young people in sensitive subject matters in RSE can be challenging, and using Serious Gaming technology may support young people and educators in this process. This paper describes the use of Intervention mapping (IM) in the development of a serious game on the topic of sexual coercion for use in RSE. IM is a process that draws on stakeholder engagement and the theory and evidence base to support health improvement intervention planning. Serious game developers transformed the game concept 'flat plan' into an interactive gameshow. The game is teacher led and aims to engage students in game play and discussion around the issue of sexual coercion. The final product known as PR:EPARe (Positive Relationships: Eliminating Coercion and Pressure in Adolescent Relationships) is the subject of an ongoing cluster Randomised Controlled Trial (RCT) in local schools. Early data analysis shows improvements in psychological preparedness for dealing with sexual coercion against some change objectives. This work represents the first attempt to use IM in the development of a Serious Game and the use of Serious Gaming for RSE delivery. RCT work is ongoing and PR:EPARe will become part of local RSE delivery in the new school year. Plans for ensuring broader impact of the game are in development.