"Different Worlds - Part One" is the 543rd episode of Casualty and the first episode of series 21. It was directed by Patrick Lau and written by Ann Marie Di Mambro. It is the first part of Casualty's 20th anniversary special.
The episode is split between scenes in the emergency department and scenes in Cambodia and features a guest appearance by Cathy Shipton as Lisa "Duffy" Duffin who had last appeared on the show in 2003.
Charlie, Comfort, Guppy and Abs arrive in Cambodia to volunteer at a medical clinic set up by Mike Hornsey and Duffy — a former Holby City ED nurse. Charlie is delighted to be reunited with Duffy after many years but is taken aback when she asks him to stay in Cambodia to help out with the clinic. Meanwhile, Guppy and Abs take time to explore Phnom Penh and meet an eager young boy named Chan.
In Holby, Maggie is tasked with introducing Somnang Lim — a Cambodian doctor who works with Duffy — to the world of British emergency medicine. He is in awe of the services the NHS offers in abundance, but he struggles with the cultural barrier. While he and Maggie work together well while treating a patient with a burst gastric balloon, Somnang ends up provoking the ire of Harry after espousing his traditionalist views on death following the fatal overdose of a young patient.
In a village on the outskirts of Phnom Penh, Charlie, Duffy and Comfort meet a young woman named Voleak who is HIV-positive and fears that she will be ostracised. However, their attentions soon turn to Chan who has suffered an extradural haematoma after falling from a coconut tree. Miles away from the nearest hospital, Guppy calls Harry who talks him through making a burr hole to relieve the pressure inside Chan's skull; Guppy successfully performs the procedure, saving Chan's life. Charlie grows suspicious of Mike after he refuses to operate on Chan, but Duffy takes umbrage and wishes that Charlie had never come to Cambodia. Elsewhere, a hopeless Voleak leaves the village with a rope.
Back in Holby, a fight breaks out on a bus which distracts the driver and causes her to crash.
- Derek Thompson as Charlie Fairhead
- Martina Laird as Comfort Jones
- Elyes Gabel as Gurpreet "Guppy" Sandhu
- James Redmond as John "Abs" Denham
- Simon MacCorkindale as Harry Harper
- Susan Cookson as Maggie Coldwell
- Janine Mellor as Kelsey Phillips
- Georgina Bouzova as Ellen Zitek
- Ian Bleasdale as Josh Griffiths
- Kip Gamblin as Greg Fallon
- Suzanne Packer as Tess Bateman
- Luke Bailey as Sam Bateman
- Sam Grey as Alice Chantrey
- Elizabeth Carling as Selena Donovan
- Ben Price as Nathan Spencer
- Cathy Shipton as Lisa "Duffy" Duffin
- John Bowe as Mike Hornsey
- Bounsy Luang Phinith as Somnang Lim
- Denise Black as Grace Anning
- Zac Fox as Jason Anning
- Moya Brady as Liz Craddick
- Stephen Marcus as Tommy Indler
- Rory Jennings as Kevin Bingham
- Danielle Tabor as Jade Grimes
- Priyanga Burford as Maria Jefferson
- Katie Walker as Claire McPherson
- Natalie Taylor as Daisy Hillier
- Leilah Isaac as Sadie Hatchard
- Tex Jones as Shorty Boyd
- Dominic Baron as Damien Rawlings
- Muriel Barker as Mavis
- Chan Thorng as Any Lim
- So Kounthi as Chan Prach
- Ip Sophorn as Voleak Chhay
- Teb Rindaro as Sonak
- Hourt Sithan as Villa Sok
- Om Sakhna as Chan Prach's mother
- Kong Bopha as Chan Prach's sister