Electronic games delivered via smartphones have the potential to become valuable tools in HIV prevention in high-prevalence and low-resource international settings. To ground theoretical elaboration around novel mHealth interventions in contextual realities, it is important to understand the mechanisms of their effects as perceived by local populations. Such perspectives are particularly important when working cross-culturally. 'Tumaini' is an interactive narrative-based smartphone game that uses a 'choose-your-own-adventure' format. It is designed to prevent HIV among young African adolescents (aged 11-14) by increasing age and condom use at first sex. It was developed with a US-based commercial game developer and is grounded in social behavioral theory, evidence-based practice and contextually relevant scenarios. In a 2017 randomized pilot study (n = 60) in Western Kenya, 'Tumaini' showed promising effects on behavioral mediators of sexual debut. In subsequent focus group discussions, adolescent participants and their parents shared their perceptions of the game's mechanisms of effect, which included motivation to play, future orientation, decision-making, relationship to a diverse range of characters and a bridging of the virtual and real worlds. These findings align with our theoretical framework, confirm its successful translation into the intervention and will inform mediation analyses in an upcoming efficacy trial.