DEERING, VIRGINA – In a very run-down hospital, there are multiple patients sick with advanced flu-like symptoms. When the local doctor, Dr Forrest, dies, his secretary Lillith tells Sheriff Mills that it's time to get help…
Connor briefs the team, in this very small town, there are three dead, with another twelve presenting symptoms. Durant lists the symptoms: fever, chills, headache, muscle and eye pain. The lungs fill with fluid and the patient dies of respiratory arrest. The onset is fast, 1 to 2 days and 2 to 3 to death. The town is isolated so hopefully the outbreak is contained, for now.
As the team drives into town, they note the town seems deserted. All the shops are closed, no people in the streets. Connor says that before he died, the town doctor quarantined anyone presenting symptoms to an old hospital on the edge of town. The hospital's been closed for about four years, the population of the town dwindled and it was no longer possible to keep it open. Only Dr Forrest stayed. Sheriff Mills says that more than twelve are sick now, which is about 10% of the population. Connor orders gloves, gowns and masks for everyone working with the sick. He tells Sheriff Mills to get his deputies to go door-to-door for people who are sick.
A woman, Emily, comes in with her young sons who are sick. Connor checks them and notes their lymph nodes are swollen. Durant protests that the sick should be moved to a real hospital but Connor argues that right now the situation is contained. This contagion is probably person to person so here it won't spread. Durant asks Sheriff Mills to see the three dead. Sheriff Mills put photos on each body to show what they looked like before they died, the dead all have mottled black markings on their faces and neck. Connor tells Durant to take the bodies to NIH, do post-mortems. Durant notes a cut on the first victim, a lawyer, Mr Gulman's hand and it was infected.
Connor interviews Lillith, discovering that she transcribed Dr Forrest's notes so she can help. The cut on Gulman's hand came from an antelope he killed on a hunting trip in Africa. That was last month but he was into taxidermy and cut his hand on the antler. Connor puts Miles in charge while he investigates Gulman's house, taking swabs from the antelope. Connor is interrupted by an old black man, Stanley Price, who is the caretaker. After explaining who he is, Connor questions Stanley and his wife about Gulman. He got sick about a week ago, even throwing up in Stanley's truck. He was sick for three days before Stanley's wife Iris returned from visiting her family. Dr Forrest took him to the hospital and Connor knows the rest. Durant calls Connor to tell him that the virus is Influenza A but no identifiable subgroup, this is a mutation, a completely new strain. Miles calls Connor to return to the hospital and Connor arrives in time to watch a patient, Donnie, die.
The team has a meeting, with Durant by phone. Flu mutates every year which is why there is a new vaccine every season. The worst flu was in 1918 called Spanish Influenza which killed millions but the word of it's impact was minimal because it was during WWI. It disappeared as quickly as it arrived. Eva reports that there have been no reported cases outside Deering, the flu is contained. Durant reports that the cut on Gulman's hand was infected from bacteria from the antelope but it wouldn't have mutated the flu. Miles grimly adds that Donnie died within 48 hours of presenting. Connor tells Eva to trace the movements of the first three cases – the lawyer Gulman, grocer Tucker, and Dr Forrest – and look for commonalities. Stanley brings in his wife Iris, who is now sick. He admits he was wrong not to listen to Connor, he should have brought her in yesterday.
Connor and Frank go to Tucker's house and after searching the house and store, they find dead chickens in a coop in the backyard of Tucker's house. Connor mentions that the Hong Kong flu in 1997 mutated from chickens, they had to kill a million chickens to stop it. Spanish Influenza also mutated from fowl. They must ship the chickens to Durant at NIH.
Miles spends time with Lillith who tells him that 90% of the people in this town were born in this hospital. She left at 18 but returned when her dad got sick. Miles mentioned that he understands leaving, his dad says he's a liberal but spends all his time making money.
Connor checks on Iris and Stanley says he thought that with all the shots, vaccines and pills around these days, this couldn't happen. He also mentions that his mother told him stories of communities becoming ill when he was a boy. Durant calls to report the chickens died of dehydration, they're not the source. Lillith and Sheriff Mills are now sick. Miles rebukes Lillith for not saying something sooner but she insists she wanted to help. Connor notes with concern that Miles helps her into a bed and tucks her in.
The team has a meeting. Eva reports she's found something. She's created a map of the town, using colored strips to trace the first three infected's movements. She found that if the client couldn't come to them, they went to the customer, in essence, the three spread the infection house to house. There was one house where all three went frequently but that person's not here. Her name is Michelle Carpenter and she's dead of brain cancer, three weeks ago. At least, that's what the assumption was. Connor and Eva go to get an exhumation order. They get a judge in a nearby town to give the order. Eva even has the cemetery manager on speed-dial.
Connor and Durant do the autopsy, realizing the flu killed her. But they find radiation implants in her brain, an attempt to stop the tumor and realize that the radiation is the source of the mutated flu. Then the first three then got infected while visiting her. But even though they now know the source, they don't have a way to stop it.
Miles hovers over Lillith, who is rapidly deteriorating. Connor returns to Deering and reports to Eva and Frank, who tell him that no-one infected left town. Connor feels like he's missing something and reviews what they know, realizing what has been in front of him all along: Stanley. Stanley was around Gulman for the entire time he was sick, then around Iris and is here at the hospital, but he's still not sick. Connor talks to Stanley about his medical history and realizes that Stanley had Spanish Influenza in 1918 when he was 2 years old. He had a natural immunity to it, and since he hasn't gotten sick now, he must have an immunity to this flu too. Connor gets Stanley to NIH and Durant confirms his antibodies work very fast on this flu. She can only make three vials of anti-serum, but can make far more in 48 hours. Unfortunately, 48 hours will be far too long for most of the sick. Connor tells the team they will have to ration the anti-serum, and Miles becomes very upset, how do they decide who lives and who dies? Connor says they leave out those who will survive until the next batch is ready and those who are already too far gone. Realizing that this includes Lillith, Miles protests that Iris is the oldest patient whereas Lillith is only 24, why should Iris live while Lillith dies? Connor patiently points out that this is triage, they have to use the serum on those most likely to survive.
Miles visits Lillith, who has heard about the serum, and she understands, telling him to make sure Emily's two boys get the serum. She also tells him to talk to his father before it's too late. The serum works quickly and people are soon walking around, being discharged. Connor finds Miles outside, obviously depressed, and commends him for good work. Connor shares a story about how when he was about Miles' age, he could figure out any disease shown to him. When someone very dear to him came down with a bacterial infection in the lungs, Connor declared to the doctors that he could cure it. But he couldn't, it defied everything thrown at it. Nature doesn't care how much training they have, either Miles must learn to deal with it or quit, and he's too good a doctor to quit. Emily and her sons leave the hospital, thanking Connor and then Connor walks off. Miles calls his dad, saying he wants to come home to visit.