The research question of the paper is How can financial firms go green? Change in banks, fund managers and other financial firms is at the heart of green finance, its role in green economy changes, and rapid responses to significant risks of climate change. This involves purpose led change in complex systems in firms and comprises significant problems of understanding and action. Complexity must be addressed to manage change and problems. This requires closing knowledge gaps in practice and academy. Field and archival research are used to reveal the main elements, connections, and interactions, in the case financial firms as complex socio-technical systems (Mitleton-Kelly (2003), and in their green change process. The focus of this empirical narrative is on change in nonfinancial aspects and how this changes financial activities. An interdisciplinary approach (Knights and Willmott, 1997; de Bakker et al, 2019) is adopted to interpret the resulting empirical change narrative and develop an equivalent theoretical narrative. These narratives constitute a conceptual framework in the form of a Green ‘Behavioural theory of the financial firm’ (green BTTF). This knowledge strategy (Zack, 1999) directly addresses issues of uncertainty and complexity by closing the knowledge gap (Holland, 2010) between, what academics and practitioners know about the greening of financial firms, and what they need to know. It is a means to close knowledge gaps in practice and academe in co-ordinated ways. This is a basis to develop integrated thinking and promote holistic actions about knowledge dynamics involving green change in the firms. This holistic narrative approach has potential ‘to make a difference’ in; learning, knowledge dynamics, thinking, and believing about desirable actions and responses to climate change (Shiller, 2019; King and Kay, 2020) in financial firms and wider systems. These are part of the evolving means to realign value in financial markets with values of wider society (Carney, 2020).