Aim(s): Play contributes uniquely to effective learning and the development of children's socio-emotional skills. This study investigated "Book of Beasties": A school-based, socio-emotional intervention centred around a card game which aims to improve children's emotional literacy and wellbeing through playful learning. Exploratory research was conducted to determine its potential as an effective resource for school staff and other professionals working in school settings with responsibility for supporting pupils' mental health. Method: A single case study comprised one London primary school. Four children (aged eight to nine years; two boys and two girls) received the intervention. Qualitative data were collected from session observations, a focus group with children and semi-structured interviews with school staff (the delivery agents) and parents/carers. Data were collated and thematically analysed. Findings: Integrated findings suggested intervention recipients had benefitted in terms of increased socio-emotional skills and wellbeing. Child and adult participants agreed that "Book of Beasties" was valuable and acceptable and thus socially valid. Specific components which contributed to the intervention's effectiveness were elicited. These included fantasy elements of the game and sensory-focused activities. Limitations: Data were collected from a single, volunteer school. The class teacher (delivery agent) selected the children who received the intervention, potentially biasing the findings. Conclusions: Exploratory research indicated that "Book of Beasties" was a promising resource for practitioners in schools and can contribute to the evidence-based socio-emotional literature. A full evaluation using both quantitative and qualitative methods was recommended to examine intervention effectiveness on socio-emotional outcomes, process issues and potential adaptations.