The Doctor Who episode "Let's Kill Hitler" is about Amy and Rory's friend, named Mel. Mel hijacks the TARDIS, which is the Doctor's time-traveling spaceship, and forces the Doctor to travel back to Nazi Germany. The purpose of this trip is to kill Hitler. The summary also mentions that Mel later regenerates, which is a process where Time Lords (like the Doctor and apparently Mel) change their physical appearance and personality after facing a fatal injury
