The story of Morell Mackenzie and his involvement in the case of Crown Prince Frederick III (the future Emperor of Germany) is as well known as it is controversial. The consequences of the case were profound, both medically and politically. Most documents concerning the case are affected by varying degrees of bias, and as a result our understanding of the true events is incomplete. We present a brief summary of the case, and review an unpublished manuscript which adds to our understanding of the events. This manuscript is supportive of Mackenzie's early management of the Crown Prince's illness and acknowledges the importance of the case in medical history.