Research on the commercial interface in inter-organisational projects has developed in recent years but still has weaknesses, principally due to its theoretical reliance on transaction cost economics (TCE). We address those weaknesses by providing an innovative intervention-based research (IBR) study of owner commercial strategy development for a complex project that goes beyond TCE to provide a pluralistic perspective. We show how this pluralistic perspective, which we dub the four forces model, provides the principles for the development of commercial strategy for managing the commercial interface by project owners. We then show how the owner's commercial strategy evolved in the face of capability constraints as it moved through the project lifecycle. We thereby contribute to theory and practice in project organising research first by situating the commercial interface between the owner and the supplier domains of project organising as a central concern in project organising research; second, by providing an empirical basis for the strategic management of the commercial interface by project owners; third by developing a pluralistic perspective on managing the commercial interface from a project owner point of view that moves beyond the current reliance on TCE theory; and fourth by introducing IBR as a novel research method.