It is the second part of a two-part crossover with Holby City's sister show Casualty. It guest starred nine regular characters from the latter show, including Connie Beauchamp, portrayed by Amanda Mealing, who was previously a regular character on Holby City.
- Guy Henry as Henrik Hanssen
- Jaye Jacobs as Donna Jackson
- Marcus Griffiths as Xavier "Zav" Duval
- Catherine Russell as Serena Campbell
- David Ames as Dominic Copeland
- Dawn Steele as Ange Godard
- Alex Walkinshaw as Adrian "Fletch" Fletcher
- Rosie Marcel as Jac Naylor
- Bob Barrett as Sacha Levy
- Hugh Quarshie as Ric Griffin
- Kaye Wragg as Essie Di Lucca
- Camilla Arfwedson as Zosia Self
- Francesca Barrett as Beka Levy
- Naomi Katiyo as Darla Johnstone
- Amanda Mealing as Connie Beauchamp
- Jason Durr as David Hide
- Michael Stevenson as Iain Dean
- Rebecca Ryan as Gem Dean
- Neet Mohan as Rashid Masum
- Shaheen Jafargholi as Marty Kirkby
- Jaye Griffiths as Elle Gardner
- Tony Marshall as Noel Garcia
- Maddy Hill as Ruby Spark
- Emma Curtis as Holly Cartwright
The episode was produced by Rebecca Hedderly. In a blog post written by Casualty and Holby City executive producer Simon Harper in February 2019, he revealed that the idea of a crossover was pitched to the BBC by the Holby City senior team in early 2017. A more detailed pitch discussing the logistics of the crossover was given to the corporation in spring 2018.
Initial drafts of the crossover episodes depicted a large vehicle crashing into a electrical substation, causing the power outage; it was later changed to a cyberattack after details about the crash didn't add up.
Filming of the crossover episodes took place in autumn 2018.
On 15 February, a BBC press release revealed that two crossover episodes between Holby City and Casualty would air at the beginning of March. It also revealed that Amanda Mealing, Rebecca Barrett and Naomi Katiyo would reprise their roles as Connie Beauchamp, Beka Levy and Darla Johnstone respectively. Later that day, a 40 second trailer was released, showing scenes including the cyberattack on the hospital and Sacha's car crash. On 21 February, the episode title and full cast list were released. On the same day, a 20 second trailer was released for the second part of the crossover, showing Henrik walking through the hospital corridors in the dark. The shows' social media teams also created the hashtag '#CasualtyXHolby' for the crossover to increase its media profile.
In the build-up to the first episode airing, several Casualty and Holby City cast members were interviewed about the crossover. On 27 February, Bob Barrett and Cathy Shipton appeared on BBC Radio 2; the following day, Shaheen Jafargholi and Rosie Marcel appeared on the ITV show Lorraine.