"You Make Me Sick" is the 999th episode of Casualty and the 38th episode of the 30th series. It was preceded by "The Best Day of My Life" and followed by "History Repeating". The episode was directed by Julie Edwards and written by John Yorke. This episode features the final appearance of MacKenzie "Big Mac" Chalker portrayed by Charles Dale who first appeared in November 2007. Although it's unclear and not directly revealed, the episode was set out to show that Big Mac's events of the day were a flashback and that him speaking at the NA meeting at the start was the same meeting as at the end.
Big Mac is at the NA meeting, and begins to tell the others of how he lied to everyone, and he begins to tell them his story. Flashbacks show him arriving for work earlier that day, and is immediately interrogated by Noel who suspects that he was the one who took the pills, not Charlie. Big Mac continues to deny it, so Noel questions as to why he isn't doing more to find out who the real culprit is. Shortly after, a man named John turns up at reception angrily inquiring as to whether his daughter Jennifer has been admitted, as he suspects she may be at risk driving a car when she hasn't learnt. Noel takes her picture and tells John that he'll let him know if he sees her or she's admitted. Big Mac also sets up a security camera in the staff room and leaves a bottle of Vitamin C tablets on the side to tempt the thief, despite knowing full well the thief is himself.
Meanwhile Jennifer is in fact driving around Holby, dangerously swerving around passing cars. Elsewhere, Phoebe allows her son Alex and his friend Joe go out driving, but as long as Joe drives. They agree, but Alex takes over and insists on driving. Jennifer loses control of her car, and crashes into a parked car round a corner, and as Alex and Joe turn the corner whilst speeding, they smash into the side of her car. All three of them are admitted to the ED. Rita relieves a call from the police who inform her that it wasn't Mark who slashes her tyres and they presumably tell her who it was.
Meanwhile Jennifer, Alex and Joe are admitted. Alex and Joe are both being treated, but Joe takes an unexpected turn for the worse and dies. However, confusion occurs as they'd previously swapped names so Alex could say that he was Joe and he wasn't driving. Alex's mother is called into the ED and informed that Alex has died and she's distraught. Mac takes her down to the mortuary, but she receives a call from Alex and she's furious with Mac for lying to her despite his attempts to explain the situation. It's eventually resolved and Rita lets Mac off the hook due to the fact the story was a result of the boys' lying. Alex eventually decides to own up and admit to Joe's parents that he was the one driving and that it was his own fault.
Robyn and David are preparing for their next mortality café, and Robyn is preparing to meet up with Glen across the road at The Hope & Anchor at lunch. Elle enters the staff room, and almost everyone leaves, still angry with her for getting Charlie suspended. Dylan tells her that she should try to find who was the real thief in an attempt to repair her relationship with the rest of the staff. Noel tells Dylan that Jennifer's father John wants to see her, and he takes him to where she is in cubicles. Big Mac later sees Jennifer outside the ED and she explains that John's her step-father and has abused her, and that she needs to get to Clacton where her aunt lives. Big Mac agrees to take her and leaves to go back so he can inform them that he's going off shift.
Big Mac returns to see Jennifer with Ethan and Rita which leaves Jennifer feeling intimidated and she pushes Rita and runs back into the ED in panic. She sees John, and runs into the staff room and in the commotion pockets the bottle of Vitamin C Mac left on the side. Rita later discovers her in the staff room and calls for assistance but Mac intervenes after seeing the CCTV and informs them that it was just Vitamin C and explains himself. Rita takes John into the office, but when she goes back to check on him he's gone. He has taken Jennifer back outside and shouts at her whilst dragging her away, and attributes it to her own mental health problems. Big Mac intervenes when he pushes her over. Despite being punched and kicked by John, Big Mac is able to prevent Jennifer from getting hurt and John is later arrested. Robyn meets Glen at the pub and they share an awkward kiss which embarrasses her and she leaves.
After their shift, Iain asks Rita about if she found out about the person who slashes her tyres and she tells him that it was Mark. Mac comes to terms with the truth and comes clean to Rita and plans to resign. Although she tells him he doesn't have to, he does anyway and leaves the department after saying goodbye to everyone including Noel. He rides off on his motorcycle and he's back in the NA meeting, therefore showing that he was simply telling the story of the day and is still at the same meeting. It transpires that Charlie is also there and Mac is also telling him the story. He apologises to Charlie and Mac leaves the meeting as Charlie looks back proud of him.
The episodes received generally positive feedback on social media from fans, with many praising Charles Dale's portrayal of Big Mac and his time on the show. Despite this, the episode also left many people annoyed and wondering who actually slashed Rita's tyres as it's never confirmed in the episode.