"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.
Charlie, Guppy, Abs and Comfort head to Cambodia to help out Duffy. When a young boy asks for help with his sick father, it sparks a chain of events that put Guppy's surgical skills to the test and prompt Charlie to raise suspicions about the clinic's surgeon, Mike. Meanwhile, back in England, Maggie and Harry are learning to work with a new doctor on exchange from Cambodia. When a young man is brought in, suffering from an overdose, a call from Cambodia forces Harry to make a decision that will have fatal consequences. As tensions run high in Holby, things take a turn for the worse when a bus full of schoolchildren crashes and topples over.
- Main article: Different Worlds - Part One/Gallery.