This paper explores the performative role of marketing knowledge in advertising planning. It is based on an ethnography within the account planning department of a London advertising agency as it worked closely with a client and a market research agency to develop a new energy/health drink for launch in the United Kingdom. The case provides detailed support for the idea that marketing (and other) theories help perform or bring into existence that which they purport to describe. It also shows how such theories are hybridised and streamlined in the iterative battleground of a creative process, framed by the need to create plausible consumption and competitive relations. The main contribution of this article is to show how the performative power of marketing knowledge is ultimately determined not by its ?truth value?, but by its ability to produce compelling stories that stabilise the reflexive relationships between product attributes, consumer profiles and competitive relations.