This manuscript reports a longitudinal case study of how one woman, Sara, who had previously considered dropping out of high school, authored strong mathematics and science identities and purposefully exhibited agency through her experiences in high school science. These experiences empowered her to choose an engineering major in college; however, her introductory university engineering experiences ultimately pushed her out of engineering. Drawing on critical agency theory, we argue that by paying careful attention to how and why women author their identities and build agency through their experiences in high school, we may gain insight into why women may choose an engineering path in college. Additionally, we examine how Sara's perceptions of engineering structures and practices chipped away at the critical engineering agency she developed and caused her to leave engineering after her first year in college.