"How Soon is Now?" is the 603rd episode of Casualty and the 13th episode of the 22nd series. It was preceded by "Strangers When We Met" and followed by "Inheritance". This episode sees the arrival of MacKenzie "Big Mac" Chalker portrayed by Charles Dale, who remained in the show until 2016.
With Ruth struggling to treat her patient she asks Adam to step in and with a little prompt she reaches the correct course of action. The patient is then stabbed by his brother in resus and dies from his new injuries. The police turn up and begin to interview all of the staff. Charlie turns up late for work after dropping Louis off at school. Harry tells him that he is not in a position to ask for favours after he asks for their meeting to be rescheduled. After smoking in the school store room, Louis and his friends accidentally set fire to the school. With resus closed because the police are examining the crime scene from earlier, Harry decides to upgrade HTC into the temporary resus. With the help of Big Mac, Toby manages to successfully unmask the true culprit behind toxicshark.com, T.C.
Toby is forgiven of the false accusations and allowed to do proper doctoring duties instead of cleaning and laundry. After a gas canister explodes in the storeroom the teachers begin to evacuate. As they are trying to escape they are greeted by a fire door with a chain around it, forcing them to turn back to look for a different route out. Greg is first on the scene and despite Dixie's orders he enters the burning building, saving a whole classroom full of pupils. After hearing more screaming from inside the building, Greg ventures back in to find a girl impaled by a plank of wood. While treating her he begins to succumb to the smoke and he is then saved by the fire brigade. Charlie starts to think that the worst has happened to Louis after there has been no sighting of him leaving the building. Charlie finds out that Louis is safe as he skipped school after starting the fire, leaving every other pupil in the path of the impending fire. Adam calls Alice 'squeak' one too many times and she finally tells him her true feelings, about how he is being insensitive to both staff and patients. T.C. and Kelsey are put on punishment duties after Kelsey helps T.C. set up a prank to apologise for his recent blogging which causes Harry to have to lay down the law.