A bike courier arrives at the Vitrus Petrol office building. He goes inside. He gets into an elevator with an older man who is sweating profusely. The man stares at the courier, his nose bleeding, and tries to staunch the flow. The elevator doors open and the man gets out and walks away without signing in. The secretary tries to stop him but the man walks down the hallway and then collapses. The courier and the workers apply CPR but it isn't successful. The dead man's veins suddenly bulge all over his body and he emits a spray of blood from his mouth.
At the Boston Children's Science Center, Astrid tries to find Walter and explains to an assistant that Walter is lost. Meanwhile, Walter is explaining the first circumnavigation of the globe by Magellan to a group of children, and how only 18 men survived. He tells them how if a monster under their bed kills them, they should check first so their screams alert their family. Walter's membership is revoked and Astrid leads him away. She gets a call summoning them to the Vitus building.
Peter and Olivia arrive at the office building first and Detective Kassel, the officer in charge, gives them the ID of the dead man, Radjan Vandenkemp. Olivia talks to the secretary, who explains that no one was expecting Vandenkemp and he didn't seem to know anyone there. As Broyles calls to tell them he's outside, Vincent Ames, the VP of Geotechnical Engineering, comes over to ask if they'll be released soon. Olivia warns that she doesn't know and informs him they'll let everyone go as soon as they know. The secretary comes over to direct Olivia to the courier, Mike, but they discover he's stepped away.
In the bathroom, Mike is washing his face and discovers that he's bleeding.
Astrid and Walter arrive and Broyles greets them and explains what happened. Walter has several theories and discusses them as they arrive at the door. The courier arrives at the door and Walter quickly shuts it as he notices the man's veins are bulging. The man emits blood and collapses, and Walter insists they need to quarantine a building. As Peter arrives, Walter insists that they have to seal the building.
Walter calls to Peter and asks for more information on Vandenkemp, including who he was coming to see. The CDC arrives and Walter tells his son to be careful. Astrid tries to reassure Walter, who insists on going to work immediately.
Olivia and Peter tell the employees that they're quarantined and that Mike was dead. The employees start to panic, wondering if they've been infected. Detective Kassel shut down the air circulation to limit the spread of the virus, and Olivia separates those closest to Vandenkemp.
Arnold McFadden, field director of CDC, meets with Broyles and has his men clear out everyone in a two block radius. Astrid comes over to report that Walter has been arrested for crossing the security line. He wants to get back to his lab with his samples, and snaps at McFadden. Broyles warns that it's not helping and Walter apologizes. He then explains that they need to figure out why only the courier is showing symptoms. Broyles speaks up on Walter's behalf, and McFadden has his men Hubert escort Walter to the lab.
Ion the building, Olivia checks two months back in the appointment scheduled but finds no indication of Vandenkemp. Peter asks if she wants to call her sister to let her know what's going on, but Olivia ignores him and shows him two cancelled appointments for Ames. Olivia points out that Ames is still there despite the cancellation.
A worker calls them over and tells them they have a problem. They check on the secretary, who is bleeding from the nose. Peter has her lie down in a room as the other employees come over. They note that she was nowhere near Vandenkemp when he died. Peter suggests that the courier infected her. Olivia talks to Ames in private and tells him she knows that Vandenkemp was there to see him. Ames admits that Vandenkemp came there to sell them information from a recent reserve find that he stole from a competitor. Vandenkemp never said what competitor or oil reserve, but he claimed the information was valuable. However, there was nothing on Vandenkemp's body.
Broyles checks on Vandenkemp and learns that he was an oil consultant who flew in from Dibala. None of the passengers show any signs of infection, and neither did the people at the hotel he stayed at.
At the lab, Walter tries to isolate the virus strain and explains that viruses display certain personalities. They're only function is to survive and replicate, and use humans to do so. Hubert confirms the strain is too fragile to be separated out, and Walter concludes the virus isn't airborne or more people would be infected. He suggests they get a better sample.
Olivia tells Peter that she didn't try to call Rachel because she's already been through so much with Olivia in the hospital. Olivia insists that they'll get out alive, and Peter notes that it's so much like Olivia that she wants to protect people. He asks why she won't talk to people about her problems. Olivia notices that the secretary has come out of the room. They follow her as she walks down a hallway. She suddenly leaps at Peter, who falls back onto Vandenkemp's blood. The secretary then leaps through a window and falls to her death, blood spewing from her mouth. Olivia arrives and realizes that Peter has been infected. As he runs off to the bathroom, Broyles has the body sprayed with anti-contamination foam and calls Olivia to find out what happened.
Peter cleans himself off in the bathroom as best he can. He then runs to Vandenkemp and searches him, insisting he now has nothing to lose. He goes through the dead man's pockets and finds rental car keys. Peter figures that Vandenkemp left anything valuable in his car in neutral territory. They get the keys to the CDC, who find the car nearby and locate a suitcase in the trunk.
Walter arrives and explains to Broyles that the virus spreads itself by infecting other organisms. Vandenkemp came to the building and spread the virus, but Walter explains that it realizes that it's trapped. Both the receptionist and the courier tried to leave the building, and the virus forced them to act.
The CDC finds a drill core sample from an exploratory oil dig inside the briefcase. They take a sample from it and confirm that the virus came from it. They confirm that the soil sample was stolen from the Solum Oil Corporation. Vandenkemp smuggled it out without taking appropriate precautions. Once Walter confirms the sample came from 10 miles down, he concludes it's 75,000 years old, and responsible for the death of mammals during the Ice Age. Walter says he might be able to produce a sample, and right now they need to keep the people safe.
Walter and Hubert dons protective suits and go inside, and split the employees into two groups. Meanwhile, McFadden has his assistant call the State Department for approval on a Level Six eradication. He warns that if anyone gets outside, they have no way to contain the virus.
As Walter collects samples, Peter notices that he's bleeding from the nose. Walter has determined a way to confirm infection and starts by taking a sample from Ames. He proves clear and Astrid and Walter check Peter and Olivia next. Olivia proves uninfected but Peter secretly switches the sample swab so he appears uninfected as well. Walter has Peter take the first group downstairs and the CDC scan them with UV light to confirm that they're clean. They notice is bleeding and restrain him as he tries to get out.
One of the employees, Steve, turns up infected and they take him away to confinement.
Peter pounds on the door and begs Olivia to release him. As Walter and Astrid finish the testing, Olivia calls to tell Astrid that Peter was infected after all.
McFadden is discussing containment procedures with Broyles and insists that Level Six is the only option. Olivia tells them that 11 civilians and Peter are infected. McFadden says that they have to remove everyone tested negative and then have the Army come in to kill the infected. He takes Olivia to the computer and shows her a computer model showing that within two weeks, most of the world would be infected.
Walter removes his suit, noting that the virus is transmitted by bodily fluids. Astrid tries to reassure him as Hubert arrives to get all non-infected personnel out of the building. Walter refuses to go and Astrid calls Olivia to explain the situation. Olivia tells her what the Army has planned, and Astrid insists they'll be fine before hanging up. Walter has Astrid drag Vandenkemp's body into the kitchen so he can dissect the body, and explains that homo sapiens 100,000 years ago somehow survived the virus.
As Walter starts to lose control, Astrid asks what she can do and Walter insists he can't let Peter die, but realizes all of the infected will perish because there's nothing he can do about it. Astrid brings up the conversation about the Magellan voyage and that he said 18 of them didn't die. She insists that the voyage shows that people can survive against all odds, but Walter isn't convinced. However, he realizes that despite the fact the virus wiped the planet clear 75,000 years ago, something must have killed it. Walter realizes that the key is ash: Mount Toba erupted in the biggest explosion in 25 million years, raining sulfuric ash down across the world. Sulfur is the key to a cure. Astrid goes through the refrigerator and finds horseradish. The glycoside in it is high in sulfur. Walter injects it into a virus sample and they confirm that the virus is destroyed.
They inform Olivia about the cure. Inside, the infected victims are trying to break free through the windows as Olivia tells McFadden about the cure. The director warns that it will take several hours to make up a cure and they don't have enough time. Broyles proposes feeding fentanyl gas into the building to knock them out, but McFadden warns that it won't work with the ventilation system shut down. Olivia volunteers to go inside and start the system back up.
As Olivia slips into the building, Peter spots her on the monitors and goes after her as she takes the service elevator to the parking garage. The team arrives with the fentanyl tanker and feeds the hose in, and Olivia comes in… as Peter attacks her. She draws her gun and explains that they have the cure, but he attacks her and accuses her of betraying him. She drops the gun but knees him in the groin and tries to grab it. Peter stops her and picks up the gun, and tells her to stay down before running off.
Outside, McFadden insists that they proceed with Level 6 eradication and refuses to give Broyles any more time. The soldiers move into the building via an emergency exit.
Olivia goes to the circuit board and turns on the air conditioning. The men pump the gas in and Walter and Astrid quickly don their helmets. In the elevator, Peter passes out as the soldiers arrive.
Walter administers his cure to the infected patients. Peter wakes up and thinks Olivia, Walter, and Astrid. He apologizes to Olivia but she assures him he wasn't himself. He tells her that he's lucky she as herself.
Walter goes outside and Astrid runs after him. She asks him what he meant when he said he can't let Peter die again. Walter simply tells her that some things are meant to be left alone and walks away.