"Holby Sin City" is the 954th episode of Casualty and the 39th episode of the 29th series. The episode is in a different format to other episodes in that it is a murder mystery theme and focuses more heavily on scenes outside of the emergency department. It was preceded by "Heart Over Head" and was followed by "If You Could Bottle It". It is loosely based on and named after the 2005 film Sin City. It is a stand-alone episode, and the events are non-canon. It gained mostly negative views from both viewers and the press. The episode was directed by Simon Massey and written by Mark Catley. The episode's events were non-canon, and this episode was the third of three non-canon episodes of series 29.
On night shift in the emergency department, the hospital's CEO Henrik Hanssen gives the team a pep-talk about improving the department. He decides to monitor Robyn throughout the night, much to her disappointment.
Iain and Dixie arrive at a tattoo parlour only to find a distraught Bonnie and her boyfriend Clyde bleeding on the floor from what appears to be a gunshot wound to the chest. Bonnie hands Iain an envelope - a suicide note. Upon his arrival at the emergency department, Ethan wants to resuscitate Clyde, but Hanssen reminds him that he can’t due to a DNR document as well as a tattoo saying 'DNR' on his chest. Knowing that Ethan won’t let Clyde die, Bonnie makes out that she and Ethan have a relationship, in front of Hanssen - hoping Ethan will be taken off the case.
Ethan is suspicious that she was the one who pulled the trigger and killed her boyfriend Clyde. She later tells him that she did in fact kill Clyde, but he doesn't realise that she has drugged his coffee. By the time he manages to report it to the police, she has boarded a plane.
Although almost 5 million people watched it, general response to the episode was poor. Comments on BBC Casualty's official Facebook page by viewers are generally negative, with many saying that they preferred the regular Casualty episodes.
The Independant went on to produce a news story on the episode, saying that it was branded as the "worst episode ever". An extract from the news story read "It's easy to see how the idea came about. When a television series has been on air as long as medical drama Casualty the urge to mix things up must be strong. Unfortunately Saturday's "Holby Sin City", an experimental episode which attempted to bring Frank Miller's heightened noir aesthetic to Holby's not-so-mean streets sparked a viewer backlash with outraged fans branding it the worst episode in the show's 29-year-run".