The quest to understand what it means to "become" a teacher and the conditions in which such aspirations can be translated into lived experience, continues to exercise teacher educators and researchers alike. Whilst the literature points towards the importance of developing teacher identity, little attention has been given to understanding the transitional processes of individuals coming into teaching at a later point in their working lives. The research reported here was part of a wider project investigating the experiences of early career science teachers. In this study we follow one mature, highly qualified science graduate as she moved through initial teacher education and beyond into her first teaching post. Drawing on theories of identity and self-verification, the analysis reveals how disparity between strongly-held archetypal teacher images and socially constructed teacher identity played out and led this teacher to step away from her new career. Fostering a strong sense of self-efficacy is suggested as critical in supporting mid-career changers in making durable adjustments to new working roles and contexts of teaching.