The STEP design space is an intimidating and hostile arena to operate with extensive uncertainties in the pathway heightened by many moving parts, causality breakdown, and multiple significant decisions to be made. Many strongly interacting elements (tensions) of the requirements/constraints and the design within the physics, technology and engineering spheres and between them makes the pathway opaque. This paper outlines the development of an integrated concept for the STEP Prototype Powerplant (SPP) making use of the of the NASA developed Concept Maturity Levels (CML) to structure and pace the work. The CMLs provide a structured progression, starting at forming an understanding of technology options. This is followed by trade space exploration where a range of concepts are developed to identify the major influences on the performance. Finally, down selecting and consolidation onto a single point design. Choosing a singular design based on experience and iterating a design ignores potentially innovative design areas. Conversely exploring every possible combination of options leads to an impossibly large number of permutations. Two actions were taken to provide direction and enable development. Firstly, the process to develop a powerplant concept was developed enabling multiple concepts to be developed quickly, with several stages representing the level of fidelity of assessments. Dependant on performance these could be further developed to greater fidelity, iterated to improve or parked. Secondly the concept development process needed to have a clear focus on delivering understanding for major physics and technology defining decisions. A subset was selected as ‘design family decisions’ based on their impact on the plant design. The concept development process was used to enable the design family decisions to be made, providing a clear basis for development of the SPP concept. Having followed this process, we now present the SPP design giving key technology options and major machine parameters.