"Different Worlds - Part One" is the 543rd episode of Casualty and the first episode of the 21st series. It was preceded by "Get What You Deserve" and followed by "Different Worlds - Part Two". The episode was directed by Patrick Lau and written by Ann Marie Di Mambro. It's the first part of Casualty's 20th anniversary special. The episode is split between scenes in the emergency department and scenes in Cambodia. In addition, it sees a guest appearance from Lisa "Duffy" Duffin, portrayed by Cathy Shipton, who was last seen in 2003.