Numerous high-profile efforts have sought to "turn around" low-performing schools. Evidence on the effectiveness of school turnarounds, however, is mixed, and research offers little guidance on which models are more likely to succeed. We present a mixed-methods case study of turnaround efforts led by the Blueprint Schools Network in three schools in Boston. Using a difference-in-differences framework, we find that Blueprint raised student achievement in ELA by at least a quarter of a standard deviation, with suggestive evidence of comparably large effects in math. We document qualitatively how differential impacts across the three Blueprint schools relate to contextual and implementation factors. In particular, Blueprint's role as a turnaround partner (in two schools) versus school operator (in one school) shaped its ability to implement its model. As a partner, Blueprint provided expertise and guidance but had limited ability to fully implement its model. In its role as an operator, Blueprint had full authority to implement its turnaround model but was also responsible for managing the day-to-day operations of the school, a role for which it had limited prior experience.