Viewers can see episode one on tonight's Casualty at 8.40pm, but will then have to wait until Tuesday's Holby City (8.oopm) to find out how the tense situation resolves.
With vital equipment malfunctioning as a result of the attack, the medics face an uphill struggle to keep patients alive. And the situation is made worse by the fact that two members of staff - paramedic Iain and consultant Sacha - are both in real danger.
Cue a battle between Connie and Jac as to which colleague will receive surgery in the one, still-functioning operating theatre.
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"It's a clash of the titans," reveals Amanda Mealing, who plays Connie. "Fans will get what they want because it's a clash of two very strong women. It's delicious, particularly because Rosie [Marcel, who plays Jac] and I used to be neighbours when we worked together in the past. So she and I are close. But then you do often find that two of the greatest enemies on screen are actually best friends in real life."
Sacha will find himself a patient rather than a doctor as a result of a dramatic car crash that endangers not just him, but also daughter Beka and Ric's granddaughter Darla. Speaking about the filming of the stunt, actor Bob Barrett said:
"It was one of the most amazing things I've ever seen. But Casualty do these stunts all the time, so they were really cool about it. Their response was: 'Yeah, it's a good stunt, but we've done better.' And then I saw the motorway crash that led to Sam's death, so I had to say, 'fair enough'. We don't really do these type of stunts on Holby City - people tend to slip over and we call that a stunt."
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As to what else we can expect from the two-parter, executive producer Simon Harper had this to say about the team effort: There is major audience appetite for a mega-crossover between these sister shows, so that’s exactly what we’re serving up!
“Both teams will strive heroically against the odds in two episodes of pure, nail-biting emotional medical drama. I’m in awe of the writing and direction and proud of both the Cardiff Casualty and Elstree Holby crews and casts in making the mind-boggling logistics work, quite aside from their creative wizardry.
“There is so much to relish in the interactions: an especially poignant thematic resonance between Connie and Jac in delicious conflict, and perhaps most vitally, who from Casualty has been secretly carrying on with whom from Holby!”