This case study describes 'First Line', a multi-year service design project focused on changing mental healthcare for young people that was conducted through collaboration between Experio Lab and Första linjen in Karlstad, Sweden. The project reflects a 'Trojan Horse' approach to changing healthcare in that service design was brought in by the clinical team based on the explicit agenda of developing new digital mental health services for young people. However, bringing in service design also came with the somewhat hidden agenda of supporting new ways of working within the context of mental healthcare. This case study tells the story of the overall project, how this 'Trojan Horse' approach to change through service design unravels and the benefits and tensions amid the complexity.