"Chain Reaction" is the 996th episode of Casualty and the 35th episode of the 30th series. It was preceded by "Hello, I Must Be Going" and followed by "This Life". The episode was directed by Jermain Julien and written by Jon Sen. It's Elle Gardner's first centric episode.
Jacob has his first shift working with Elle and in reception, they recognise each other as old friends and reunite. Connie is shocked to discover they're friends but tries to hide her resentment of their relationship. Meanwhile, a fire starts at a local chemical factory and the factory's owner Saeed is still inside. Outside in a car, his daughter Rosa and her boyfriend Philip, also an employee, notice smoke rising from the roof. They run back inside to find Saeed whilst an evacuation is in progress. Rosa manages to get her father out, but Philip is injured when an explosion takes place and he falls in the lower levels.
Jez tells Iain that he should speak to Rita about their relationship due to the fact she keeps constantly texting him. They are interrupted by the report of the factory incident, and go to the scene. Elle speaks with Henrik in the ED, and Robyn and Louise notice their friendship although they decide to keep quiet about it.
Meanwhile Dylan's father Brian Carroll injures his hand whilst smashing his car window to get to his daughter Rihanna whom he accidentally locked inside. He initially requests to be treated by another doctor, but they eventually manage to cope with each other. Hazel arrives shortly after and tries to convince Dylan that Brian is a good father and has changed but Dylan isn't convinced.
Lily is assigned to treating Saeed, and it soon transpires that his injuries may be more severe than what was originally thought. Rosa speaks to him about the incident and asks if he knows what started the fire and becomes suspicious as he was already in the basement where the fire started. She goes to wait in reception where she overheard David tell Elle that Philip is still in the factory and a doctor is required at the scnee. Jacob manages to convince Elle to go, as they set off together without Connie and Rita's permission. They are successful in recovering the patient alongside the paramedics and arrive back in the ED with Philip who has a broken femur.
Rosa continues to question Saeed about the incident but he informs her that he can't remember what happened. Rosa's concerns grow when she notices his fluid drain has blood going into it. She calls Lily in and it soon transpires that he's been taking too much warfarin which is causing his blood not to clot. After scans are done, it's also discovered that he's suffering from a form of Alzheimer's disease which is causing his memory problems although Lily assures him it's nothing serious to worry about. Rosa goes to visit Philip who informs her that the explosion was caused by someone who put acid in the chemicals, hinting at Saeed although Rosa tries to defend him. Rosa goes back to her father and assures him that everything will be okay.
Connie confronts Elle and Jacob about their decision to leave the ED without her permission, and requests that Elle go to her office. Elle tells Connie that she didn't intend to break the rules, but Connie tells her that the department only runs smoothly because it runs the way she wants it to. Connie assures Elle that she has the full support of Hanssen, although she's unaware of his friendship with Elle. Elle later praises David's work that day but keeps referring to him as Dave, something he hates.
Brian almost collapses near the exit and Dylan informs him that he has a lung condition that has been caused by his smoking, and encourages him to stop. Brian later apologises to Dylan, but he is still unconvinced and Hazel asks Dylan to be Rihanna's godfather as they plan to get her christened the following month. Despite Dylan's earlier advice, Brian goes outside for a cigarette. Dylan says that it's his own choice as to what he does but advices him to listen to him.
Jacob speaks to Elle about Connie and tells her that she's a good boss. He also tells her that he has worked out she knows people in high places. Connie sees them talking and gives Elle a cold stare and walks away. Iain meets with Rita in the pub and despite wanting to follow Jez's advice, he gives into his temptation and continues as normal. Outside the ED, Dylan speaks with Max about Zoe. Max is smoking, but he throws the cigarette away and tells Dylan he has already quit one habit recently and shows him how easily it can be done.
There's little reference to when the episode takes place, so therefore it's assumed to be on the day on which it aired. However, an email comes through from Zoe on the computer screen with a sent date of 28 May, a week after the airdate. However, in the next scene the date has changed to 21 May, so it's likely that the airdate wasn't certain during filming and therefore it wasn't known which date would be used and it wasn't changed. However, the date at the top of the tab still displayed 28 May, but this is likely just a continuity error.
The episode was originally scheduled to air at 20:45 but was postponed until 21:25 due to other scheduled programmes running over beforehand. A similar thing had happened earlier in the year with the episode "Sweet Child of Mine" when The Voice finished fifteen minutes early so the episode aired earlier than expected which meant many viewers missed the start of the episode.
Alison Graham from the Radio Times reviewed the episode, saying "Control freak emergency department boss Connie looks like she's run headfirst into a tree when she spots romantic chemistry between her ex-boyfriend Jacob and new consultant Elle. But Connie has nothing to fear. The pair are old mates, so old that Jacob, excruciatingly, refers to Elle as "Pancake". In the accident of the week there's an explosion at an ailing factory - someone's tampered with "the reactor". What is this, Sizewell B?"