It is the first part of a two-part crossover with Casualty's sister show Holby City and features 14 characters from the show, including former Casualty regular Adrian Fletcher, portrayed by Alex Walkinshaw. The events of this episode lead directly into the Holby City episode "Powerless". The events of the episode take place on 5 March 2019.
In resus, Jade becomes frustrated when the computer stops responding. Nearby, a patient appears to go into ventricular fibrillation, but she, Dylan and Charlie are perplexed when the patient wakes up. They soon become even more confused when the monitors of the other patients begin to show rapid deterioration despite being stable. The technical problems in resus also appear to affecting the whole department, preventing the staff from viewing and amending patient records. Elle calls Ric, who is also experiencing problems with the network on AAU.
Connie, still bruised and scarred by her assault, is summoned by Henrik to a board meeting with Ric, Jac, Fletch and Serena. After the meeting, they inform their respective departments that staff will have to take manual observations. When Dylan asks her about the cause of the network issues, Connie tells him that it was caused by a member of staff opening a spam email claiming to be about an NHS award; Noel overhears and becomes nervous.
At the ambulance station, Ruby notices that Iain has left his entire uniform in his locker and has removed all of his photographs. Jan dismisses her concerns, prompting Ruby to visit Iain at his house. When she arrives, he doesn't answer her shouts, and she tries to enter through the backdoor. She finds a fake rock with a combination lock on the bottom. When she successfully cracks the code and breaks open his door, she is shocked to find all of his belongings in bags for donation. In his living room, she finds him unresponsive. She carries him to her car and takes him to the ED.
Meanwhile, Darla is looking for a pregnancy test kit at a pharmacy and hides when she spots Sacha entering to pick up a prescription with his daughter Beka. When Darla accidentally bumps into Beka, Sacha offers her a lift to school. On their way out, Darla hides the test kit in Beka's bag. On the road, Sacha stops abruptly to avoid an oncoming car and causes the test kit to fall out of the bag. An argument erupts between Sacha and Beka, and he loses focus on the road. Suddenly, he crashes into a barrier, sending the car flying. Shortly after, Sacha and Darla manage to free themselves from the wreckage, but Beka is trapped and is suffering from an open fracture as well as a dislocated shoulder. Sacha tells Darla to call for an ambulance.
Back at the ED, Connie confronts Elle over the decision to move a patient to Darwin, only for Jac to interrupt and claim responsibility. As they continue to argue, Noel informs them of a vehicle entrapment; Connie tells Elle to go with the paramedics. As Elle and Jan leave to deal with the resus, Ruby arrives with Iain. Connie, Rash and David take him into resus, and Gem watches in horror as they struggle to revive him. Connie asks Ruby what she knows about his condition, and Ruby tells her that she thinks that he has deliberately overdosed. Gem refuses to believe that her brother tried to kill himself. Connie decides to treat his symptoms as a heart attack. Thinking that Connie is making a mistake, Ruby rushes back to Iain's house to find evidence of a suicide attempt. She initially finds nothing in his house, but she finds three empty bottles of pills in his neighbour's bin.
At the scene of the crash, Elle tries to calm Sacha down. As the fire service tries to release Beka from the wreckage, Elle is forced to reduce her dislocated shoulder. After two agonising attempts at relocating her shoulder, Elle succeeds and orders the fire crew to rescue her.
Ruby returns to resus with the bottles just as Connie is about to start treating him for a heart attack. When Connie accepts the bottles as evidence of an overdose, Gem storms out. Rash tries to stop her, but she tells him that he will be culpable if Iain dies. Meanwhile, as the hospital struggles to keep patients ventilated, Connie and Hanssen discuss moving patients to neighbouring hospitals. However, they are interrupted by the hospital's fire alarm.
Sacha and Beka arrive at the ED, and Dylan decides to take Beka straight to AAU with Marty so Jan can check on Iain. When they arrive on the ward, Dylan suggests immediately reducing the fracture. Sacha objects, but Dylan reminds him that he is not in control. Dylan and Marty perform the procedure, and Sacha watches in horror as Beka screams in agony. After they successfully reduce Beka's fracture, Sacha thanks Dylan and apologises. She is subsequently taken up to theatre. Outside, Elle asks Serena if Darla could stay until Ric returns from surgery. In Serena's office, Elle comforts Darla as she breaks down, claiming that she is responsible for the crash. When Elle asks her why, Darla asks her if she can keep a secret
Back at the ambulance station, Jan finds Ruby and tells her that Iain is stable. Despite telling her that she saved Iain's life, Ruby breaks down, feeling guilty for not interfering before he decided to kill himself. Meanwhile, in resus, Connie discovers that Iain has a large pericardial collection as a result of the chest compressions. Gem suggests taking him to the trauma theatre; Connie agrees. However, they cannot get in due to the locks being compromised. Connie calls Jac to ask if the theatre on Darwin is free. Upon hearing that it is free, Connie and David meet Fletch in the lift and they make their way to Darwin. Suddenly, the lift grinds to a halt and the lights go out throughout the hospital, prompting the emergency lighting system to activate.
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.
On 15 February, a BBC press release revealed that two crossover episodes between Casualty and Holby City would air at the beginning of March. Later that day, a 40 second trailer was released, showing scenes including the cyberattack on the hospital and Sacha Levy's car crash. On 20 February, the BBC released a brief synopsis of the episode, revealing that Iain Dean's life will be hanging in the balance. Later that day, the full cast list was revealed. The shows' social media teams also created the hashtag '#CasualtyXHolby' for the crossover to increase its media profile, and in the run-up to the episode airing, they created a countdown to the episode's broadcast.
On 24 February, a second trailer was released depicting more scenes of the hospital staff trying to combat the cyberattack as well as scenes of Ruby Spark finding Iain in his house after his suicide attempt in the previous episode. Furthermore, the show's official social media accounts updated their avatars to feature a glitched Casualty logo. The following episode, another promotional video was posted to the show's social media accounts, depicting a phone call from Connie Beauchamp asking all available staff to come into the ED. A similar video was released on 28 February depicting Ruby asking if anyone had seen Iain.
Four days before the episode aired, over 40 promotional images were released. Among these were images of Connie working with Jac Naylor in theatre, Iain's treatment in the ED and Sacha's car crash. A minute-long behind-the-scenes video was published on the official Casualty website which included comments on the crossover from Amanda Mealing, Lucy Raffety, Jason Durr and Bob Barrett.
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.
On 1 March, a 55 second preview clip of the first episode was released showing Connie, Henrik, Jac, Ric, Serena and Fletch gathering to attend a meeting about the cyberattack. On 2 March, five hours before the episode's broadcast, a second preview clip was released, showing Connie and Hanssen discussing the transfer of patients to neighbouring hospitals.