NOTE: This spoiler was submitted by Spectre.
We open to the sound of Beth (Gwyneth Paltrow) coughing. Title card lists it as Day 2. She is in an airport lounge when she gets a call on her cell phone. It is a man named John Neal saying she just had sex with him and left without saying goodbye. Beth says she's sorry. She was afraid to miss her flight. "Well in case I don't see you again, It was great to see you again." John says. Beth echoes the sentiment. John mentions that if she wants to contact him, she'll have to use the secure email address. She hangs up as her flight is called and she pays her bar tab.
A montage ensues where we see several other people. A young man in Hong Kong gets off a boat, sweating. He goes home to his family, and his sister looks at him concerned. In London, a model gets sick and goes back to her hotel room. Hotel staff come into the room and find her dead on the bathroom floor. On a plane, a businessman comes out of the bathroom with a pained expression on his face. He is traveling home to Tokyo. He is on a bus when he collapses to the floor, convulsing. His death is captured on a camera phone. The young man from Hong Kong leaves his apartment, coughing near several people in an elevator. He walks the streets, his vision getting more hazy. He walks into traffic and is hit by a truck. Finally, in Minnesota, Beth comes home to her husband Mitch (Matt Damon) and son Clark (Griffin Kane) and hugs her son.
Dr. Ellis Cheever (Laurence Fishburne) comes into work at the CDC. On the way in, he passes a janitor Roger (John Hawkes) who he is on friendly terms with due to the football pool they have. Roger asks Cheever about his son since the school thinks he has ADHD. Roger asks if Cheever could look at him, but Cheever says he is not that kind of doctor. However, he knows some people, so he will try to refer Roger a good doctor for his son.
Mitch goes to pick up Clark from school for what they think is a fever. Mitch tells Clark he will beat it by Thanksgiving.
At a news office, a blogger journalist, Alan (Jude Law) is talking to an editor, Lorraine (Monique Gabriela Curnen) about the Tokyo bus incident. Alan thinks it may have to do with mercury poisoning in fish worldwide. Lorraine says they don't have the budget for this kind of freelance work, but Alan says this story can go worldwide in 24 hours. Lorraine says she will pass it to another editor. Alan tells her that he recorded this conversation and if she steals his story he will sue her.
Beth is at home, getting worse. She stares at a cup of coffee and tries to pick it up as Mitch talks about his day. She instead makes it fall off the counter and shatter. Mitch comes to help clean it up, when Beth starts to have a seizure. Mitch is scared, and he sees Clark at the doorway looking at his mother dying. Mitch tells him to go upstairs as he calls 911.
At the hospital, Mitch tries to explain what happened and about Beth's allergies when she has another seizure. Mitch is pushed out of the room. A doctor comes to see him later and tells him Beth died. Mitch is in shock and asks to talk to her, not computing the news. The doctor tries to explain that his wife might have picked something up in China, but there are no health alerts there to match what she died of. The medical board is going to order an autopsy in order to determine what killed her. Mitch is aghast and screams, "What happened!"
Mitch is being driven home when he gets a call. His son is sick. He tells the babysitter to call 911. When he gets home, the babysitter says Clark had a headache, so she put him to bed. By the time Mitch gets there, it is too late. His son is dead.
In Switzerland, Dr. Leonora Orantes (Marion Cotillard) goes to work at the World Health Organization. She handles a briefing in which this unknown virus has caused cases of infection in the U.S., China, London and Japan. They need to make sure to be thorough in their investigation. She mentions Beth as being the potential start of the infection.
Back in Hong Kong, the sister of the young man that was infected has cremated his body and goes to take him away for burial via bus. A cleaning lady is checking the bus when she finds the girl dead. Her brother infected her too.
In Chicago, Beth's lover John Neal is being carried out on a stretcher. He has been infected, as well. His wife looks on in shock, wondering what is wrong.
Two medical morticians do the autopsy on Beth, opening up her skull. They notice something strange about her brain. One of the men asks if he should take a sample. "Actually, I want you stand away from the table." The other technician tells him. The technician tells his partner to call EVERYONE.
Mitch is taken back to the hospital and put under quarantine watch. His daughter from his previous marriage, Jory (Anna Jacoby-Heron), arrives. They talk on a phone. Jory feels guilty of not being there when Clark died, but Mitch says it is not her fault. In addition, if she had been, she could have been infected too. She is all he has left. Mitch tells Jory to go home to her mother, but Jory says she will not leave him. She is staying with him.
Meanwhile, Dr. Erin Mears (Kate Winslet) is being briefed by Cheever on her job for the outbreak. She is to investigate and contain cases on infection. If she needs anything, she can call him. No questions asked.
Mears gets off a plane in Minnesota to be transported to a nearby CDC office. She gives a briefing about the virus and the precaution they need to take. One woman asks if they are panicking about something they know little of. "We don't even know what to tell people to be afraid of." Mears notes that the virus is a contagion of touch. So, the littlest things that people touch and interact with, including each other are potential harbors for infection. The problem, Mears says, is the carriers and the number they will potentially infect, the R-not number. Flu is usually one person. Polio, pre-vaccine was between four and six. Until they know more, this virus' R-not number could be much greater.
Two doctors, Ally Hextall (Jennifer Ehle) and David Eisenberg (Demetri Martin) go into a secure bio lab and look at the samples taken from Beth's body among others. Hextall says they need to send it to Sussman (Elliott Gould).
Sussman is another doctor in a less secure bio medical facility than theirs. He is leaving when he is accosted by Alan about accusations that this virus is being manufactured as a potential profiting scheme by drug companies. Sussman tells him to make an appointment and insults Alan's journalism saying, "Blogging is graffiti with punctuation."
Cheever is met at the CDC doors by Homeland Security. He is briefed by their liaison, Lyle Haggerty (Bryan Cranston). Homeland Security believes the virus might be a terrorist action as it is the only way terrorists have not tried to kill people. Cheever is intrigued by the theory but unconvinced. Haggerty mentions that someone could try to weaponize Avian flu but Cheever counters that by saying, "They don't need to. The birds are doing it for us."
A news report states that the school Clark attended is being closed after three nurses and two other children died. A panic starts to form.
Mears goes to the AIMM offices where Beth worked. Many people interacted with Beth, but before she left the country, so their fears are assuaged by Mears. Mears notes none had direct contact with Beth after she came home. One remembers Aaron Barnes (Dan Aho), another coworker, drove her home from the airport. Mears calls him as he is on a bus. She tells him to get off as he is infected. He waits at the stop until Mears arrives with a containment crew.
Mitch is being asked by a doctor about Beth. Mitch knows little of substance. The doctor asks about her Chicago layover, and if Beth knew anyone there. Mitch mentions that his wife was with John Neal before they married. Realization hits Mitch. He asks if Neal is infected. The doctor is resistant but eventually nods. Mitch is stunned that his wife was cheating on him.
Hextall and Eisenberg give a briefing to Cheever about the virus sample taken from Beth's body. It latches onto the host cell, taking over completely. The body doesn't know what to do, and the virus continues to attack. They noticed there are traces of bat and pig in the virus code, so it mostly originated from an infected bat. Understanding the severity of the issue, Cheever orders Sussman to be shut down and their secure lab is the only place research will be done. Hextall calls Sussman and reluctantly tells him to shut down and destroy his samples.
Cheever briefs the CDC on the outbreak on cases of the virus worldwide. The numbers of infected reach 80,000+ and could reach as high as 267,000.
Alan continues to blog about the virus and how the government is conspiring with the drug companies to turn a profit on tragedy. He claims that a drug is available to treat the virus, but it is being suppressed by the CDC.
Dr. Orantes goes to Hong Kong. Her job is track the movements Beth made while there. Her liaison is Sun Feng (Chin Han). Sun Feng is ambivalent of Hong Kong being the focal point of the viral outbreak, but Orantes concludes it is Beth that started it all. After hearing several workers at a casino got sick where Beth attended, Orantes tells Feng to get tapes of the security footage. Feng mentions his village where he grew up is infected, and his mother is sick.
Meanwhile, Dr. Sussman defies orders and runs some further tests on the virus samples.
Hextall briefs Cheever on Sussman's findings. He was able to make a living stable version of the virus. He was able to do it by combining it with a bat viral strain they didn't have. Now that they know what they are up against, they can start testing vaccines. Cheever is pissed Sussman defied orders and asks what he wants. Hextall says he just wants credit for his work. He could have given the work to drug companies and made a fortune, but he gave it to the CDC instead.
The news report on the virus, now called MEV-1 and credit Sussman for discovering the findings. The news stresses that the government states that it will take potentially months for a full vaccine to be ready.
Alan is contacted by a news source. They want him to continue his investigations on MEV-1 and the government's alleged involvement with the problem. Alan agrees.
Back in Minnesota, still in quarantine, Mitch asks Mears if he is immune, why they cannot use his blood to make a vaccine. Mears tells him that even though his intentions are noble, blood vaccines are tricky and time consuming to make, and are not guaranteed to work.
Mears travels to an inside stadium where they will place a virus ER. Mears tells her liaison she will need three more locations like it.
Mears calls Cheever. Cheever asks if she is fine.
Mitch is at home with Jory when there is a knock at the door. It is a boy Jory likes, Andy (Brian J. O'Donnell). Andy has come to pay his respects with flowers for the deaths of Beth and Clark. Mitch says that while he is touched by the gesture, he cannot let him in because he could infect them. Jory is crushed, but Mitch tells her that they have to be very careful.
We see a flashback of Beth in the Hong Kong casino, enjoying herself with several people. She is playing a casino game when she is asked by a man, the Japanese businessman who died on the bus, to blow on his chip for good luck. She does so.
Mears wake up coughing. She goes into the bathroom and takes her temperature, praying to god that she is not infected. But the symptoms point to it. She calls the front desk and tells them to track down everyone she came in contact with to be tested.
Mears calls Cheever and tells him that she's infected. Cheever says he will do everything he can to get her home, but she should stay in her room for now. They hang up, and Cheever is stunned that Mears is infected. He sent her there, so he feels responsible.
Mitch goes to a funeral home and learns, due to his wife and son dying from the virus, the funeral home will not accept the bodies for burial. Mitch is shocked, but the funeral director says there is nothing they can do. Mitch talks to his mother in law and discusses cremation but the mother in law wants her family to be buried together. His mother in law said Beth made mistakes by cheating, but she did love Mitch. He has to forgive her. Jory texts Andy, telling him how sad her dad is.
We see another flashback of Beth in the Hong Kong casino. She calls John Neal, saying she can get an earlier flight, so they can meet up. She accidentally leaves her phone at the bar and a woman picks it up and hands it to her. It was the European model that was found dead in London. We see a table where she had sat and had drinks with several people. A waiter picks up the glasses. It is the young man that later died. That is how Beth infected them both.
Orantes notices this all from the security footage and realizes that Beth is "Patient Zero" for the entire outbreak. Orantes asks about Feng's mother. Feng says she died. Orantes says she is sorry. They go to leave, and Feng makes a call. On the way to their destination, Feng pulls her out of the car into a van. His village is dying, and he is going to hold her for ransom for the vaccine doses when they are first produced. Orantes sees the remaining villagers. Most are children.
Cheever attempts to get Mears out of Minnesota but hits roadblocks on every turn. The transport he was going to use is being used for a Congressman, as he learns from Haggerty. After that, all air travel is grounded. The president is being moved underground, and congress is working online. There is nothing Cheever can do.
Alan is live-blogging. He is apparently sick with the virus. He shows he is taking the drug Forstina, and dips it into his water and drinks it. He tells his audience that if he is here tomorrow than it worked.
Cheever speaks to his wife Aubrey (Sanaa Lathan), and tells her how bad he feels Mears is going to die, and there is nothing he can do. Cheever then tells her to leave Chicago and not tell a soul where she is going. As he hangs up, he sees that Roger was standing there the whole time. Roger is disgusted with Cheever noting everyone has loved ones.
Aubrey goes shopping for supplies when she is called by her friend who she was supposed to meet for dinner. Aubrey reveals the real reason she can't come, urging her to not repeat it to anyone.
Around the U.S., people are listening to Alan and are lining up for the drug to suppress MEV-1 that Alan has alleged works. However, demand becomes insatiable, and people begin to riot and loot all over the country.
Mitch goes for groceries with Jory in a looted store. A woman coughs nears him, and he tells Jory they need to leave. They get to the car without food. They attempt to leave the state, but the National Guard is shutting down the roads. Even though they are not infected, no one is getting through. They are forced to turn around and go back home.
Alan has been running around town in a bio suit seeing the destruction and decline of human civilization in the aftermath of the outbreak. He gets home to see Lorraine there, begging for the medicine he says helps, because she is pregnant. Alan says he has none due to a break-in, but he will get her some as soon as possible.
Meanwhile, in Minnesota, Dr. Mears is being told she cannot be moved out. She is close to death. Next to her, a man is complaining of being cold but cannot be given any more blankets. Mears attempts to comfort him and give him her coat but cannot reach far enough. It falls right next to her bed as she shudders.
Cut to Mears in a plastic body bag. She is dead. Her liaison, Dave is nearby, holding flowers, trying his best to pay his respects considering she will be part of a mass grave. Dave notes they ran out of body bags days ago, and they attempted to get some from Canada, but they were unwilling to help.
Dr. Cheever goes on TV with Dr. Gupta talking about the virus and its implications. Cheever notes coordination is a problem because each state has its own health organization and protocols. Alan is also on via satellite and when it is his turn to speak he goes on the attack, blaming the CDC and WHO for being in bed with the drug companies. Alan surmises that up to 1 billion could actually be infected and/or die before it is over. He also reveals that Aubrey's friend posted on facebook about him warning her to leave Chicago. Cheever cannot respond.
Cheever talks to Haggerty. Haggerty says he has now just become a scapegoat. Furthermore, they have no choice to investigate the claims he leaked sensitive information to his wife. Cheever nods, understanding.
Eisenberg gets an email in the lab. MEV-1 has mutated in Africa, becoming more deadly.
Eisenberg and Hextall begin clinical trials with monkeys to test vaccine variants. None work. They wind up bagging up a bunch of dead monkeys.
We see the world practically abandoned at this place. Empty streets, airports, gyms etc. Everyone is staying in, afraid.
We see posters of Alan posted around the cities. One says prophet. The other says Profit (probably signifying he too is in it for the money).
We see a shrine for Lorraine. She and her child died. Alan looks at it stoically. He apparently was unable to get her medicine in time (or possibly did nothing).
Mitch goes for an MRE handout when they announce they have run out. Riots ensue, and Mitch saves a woman from getting her food stolen.
Mitch goes home and looks outside. He hears gunshots in his neighbor's house. Looters come out. They apparently killed the family inside. Mitch tries to call 911, but gets the runaround with an automated message. He hangs up.
Mitch goes around to his neighbors to look for them. Finding none, he breaks into one of their homes and steals a shotgun for his and Jory's protection. When he gets home, he sees Jory is gone and notices snow prints moving away from the house.
Jory is with Andy making snow angels. Andy tries to kiss her saying she is immune, and he is healthy, so there should be no problem. Just before he does, Mitch comes up and pulls him off her, telling him to go home. He tells Jory to get up, and they go home.
Back at the CDC lab, Hextall is talking to Cheever. She notes that even with a complete vaccine, it would take months for it to be viable for use, and many more would die in the meantime.
Hextall notices Monkey #57 has no symptoms. It is a viable variant. Deciding to be proactive, she inoculates herself, goes to the hospital to visit her dying father to infect herself to test the vaccine.
We see another news report. The vaccine has been fast tracked for approval and production. The first batch can be ready in about 90 days.
Aubrey is at home when it is broken into by looters. They want the MEV-1 Vaccine and think Cheever has it. Aubrey says no. They had to wait like everyone else. Cheever rushes home. Aubrey was not killed. Cheever is afraid they touched her (as they are just about to receive the vaccine), but Aubrey denies it.
Alan goes to see his news contact. Alan is still adamant that the vaccine should not be used. He has 12 million people who listen to him every day. Alan sees a garbage man and gets paranoid. He runs off. He was right to do so. His contact sold him out to the police in a sting operation. He is being arrested for securities fraud and manslaughter (probably for Lorraine) among other things.
Haggerty goes on television doing a vaccine lottery. Since they do not have enough yet, they will hold lotteries on birthdays to see who gets it first.
Cheever finds Hextall on a computer and tells her to take a break. Hextall says she cannot. Not when he is being investigated. Mears died, and her father nearly died treating the sick. She clearly feels she doesn't deserve all the credit. She had plenty of help.
Back in Minnesota, Mitch and Jory watch the lottery. Her birthday didn't come up, so she is stuck in isolation for 144 days. She relates her woes to Andy.
Back in Hong Kong, in Sun Feng's village, Orantes has been teaching an art class. Feng pulls her away. It's time to make the exchange. They meet the contact in a graveyard. 100 shots for Orantes. At the airport, her contact reveals the shots they gave over were placebos and gives her the real one. The Chinese told them to do it. This kind of action has happened a lot lately, and they don't cater to terrorists. Orantes is horrified that they would condemn a village of mostly children to death and runs off.
The lead detective tells Alan they looked at his blood work. He was never sick with the virus. He lied about his condition. And now he has millions of people against the vaccination that could save them, instead using a drug that probably doesn't work at all. They are going to put Alan in jail, and his money he made (4.5 million) will be locked away too. "If I could lock up your computer, I would" the cop says. Another cop comes in. Alan apparently has made bail. Alan leaves, defiant to the end.
Cheever is presented two doses to Haggerty. One for him, one for his wife. Cheever says his wife will want to do them together. Instead, he gives one to Roger's son. Cheever explains where shaking hands originated; It came from showing you didn't have a weapon in your hand and you meant no harm. Roger thanks Cheever, then shakes his hand. Cheever puts on the vaccine bracelet to keep the charade going. He goes home and gives the other shot to his wife.
Hextall and Eisenberg go to work into the lab, putting the new virus, MEV-1 on ice, hopefully never to be seen again.
Mitch goes to a shopping mall, being stopped at several checkpoints to make sure he is immune.
Jory finds a box for her from her dad telling her to be ready at 8. Inside is a dress.
Jory goes downstairs and sees her dad made up the living room for a makeshift prom night since she is unable to leave the house yet. There is a knock on the door. It is Andy, who got the vaccine. He is going to be her date for the night.
Mitch is in the bedroom, looking for his camera, when he turns it on and sees pictures of Beth. He breaks down.
Mitch goes downstairs to take picture of Jory and Andy. Mitch watches them dance. In that moment, there is hope.
A final flashback occurs. We see what caused the global epidemic. A construction crew from Beth's company AIMM was cutting down trees. That caused some bats to fly out. One bat was infected. It grabbed a piece of banana and perched underneath a pig's pen. It dropped the banana piece which had his DNA on it. A pig ate it and was eventually slaughtered at market for food. A chef handles the pig, touching the inside of the infected pig's mouth with his hands. He goes out to dining room and poses in a picture with Beth holding hands, transferring the virus to her and starting the chain of events. A title card states it is Day 1.