A woman, Charlotte, collapses in a hotel conference room with a mysterious rash. She later wakes in an isolation tent. When asked by Elliot Taylorman, the scientist investigating the virus, if she's been in contact with anyone else and if so where they are, Charlotte says only "The Mill" before she passes out.
At The Mill, Julia takes a call to say they have a "code blue" situation, nurse Cherry is trying to take blood from an obstreperous young woman called Vic and Dr Jimmi Clay is trying to pacify a paranoid ex-prisoner, Michael. Julia is sure it is a drill but can't take any risks, so asks receptionist Karen to call everyone to make sure staff and patients stay where they are.
Dr Heston Carter is with Rowland Beckley, an ex-brigadier who is quite happy to sit and wait, while Dr Zara Carmichael's patient, Mrs Potter, gets worked up about the offers she's missing at the supermarket.
George Liston, a virologist based at the University, is called to see the woman who has collapsed - but although he clearly recognises Charlotte's condition, he claims not to. While tensions mount as the staff and patients locked inside The Mill deal with the pressure of the situation, it becomes a race against time to try to track down the source of the virus.
When Cherry develops a rash that indicates she has the virus, Dr Simon Bond goes to see George Liston for help. George is adamant there is no cure, but Simon manages to get him to reveal the medication that might save her. But will Simon be able to persuade Jimmi to break the lockdown and get the medication to Cherry in time?