"When the Whistle Blows" is the 1035th episode of Casualty and the 31st episode of the 31st series. It was preceded by "Child of Mine" and followed by "Reap the Whirlwind - Part One". The episode was directed by Tracey Larcombe and written by John Yorke.
Jez wakes up in Kalen Gardner's bed - but before long he realises that he's Elle's 17-year-old son. Eager to get away as quick as possible before she finds out, Jez escapes out the window, but falls from the gutter pipe in the process. Elle becomes suspicious of Kalen when she finds a condom wrapper in his room, and it's not long before her intrusive approach leads to the discovery that Jez slept with her son. Elle confronts Jez in the middle of the ED, but later apologises to Kalen for her outburst, and attributes it to the fact she was surprised about his sexuality. Elle also apologises to Jez, and shares a joke with him, although she didn't realise that Louise heard the whole thing. At the end of his shift, Jez was confronted by an angry Louise, who broke up with him due to his disloyalty.
Sam announces further cuts to the budget and informs Cal that he will no longer be able to do his surgical rotation. When Sam sends Dylan home, Cal proposes to Ethan, Lily and Alicia that they start a work-to-rule protest to stand in solidarity with the consultants. Sam takes them all aside when he hears of their plans and warns Cal that he'd better not mess things up if he wants to change the way things are operating. Meanwhile, Alicia is discovered when images surface on social media of her face edited onto revealing photos.
Elsewhere, a man named Stan hopes to escape the tyranny of his bullying boss by burning down a building on a secure construction site. However, he's unaware that his wife Tamzin is in the building at the time collecting incriminating evidence to use against the boss. PCs Jay and Joanna attend the scene, but Jay is worried when Joanna ignores his instructions and heads into the building to save Tamzin, who is trapped. The paramedics arrive and they manage to rescue Tamzin, but Joanna is crushed by some falling rubble and receives extensive injuries.
Back at the ED, Tamzin discovers that Stan was told by his boss to set the building on fire. Fearing that she will tell, he throws her on the floor. However, this has negative repercussions when she starts fitting and is taken back through to resus, having sustained a new head injury. Joanna dies, despite the efforts of the team, and Jay is heartbroken. Upon learning that Stan is responsible for the fire, Jay takes him into resus and tells him that the body under the sheet is Tamzin in an attempt to make him feel worse. Stan is arrested, but learns shortly before that the body is Joanna's, and Tamzin is in theatre.
Towards the end of the shift, Cal confronts Sam in reception about Joanna's death, informing him that she wouldn't have died had they not been understaffed. Cal is even more infuriated when he learns that Lily is staying for overtime, therefore breaking their solidarity. He accuses Sam of intimidating her into working, but the argument soon reaches a pinnacle as Cal threatens to start a strike and tear Sam's career apart.