A bike courier gets on an elevator with a guy who's very sweaty and has a nosebleed. They get off on the same floor, of what appears to be an oil company. The man collapses and appears to die, only to start spewing a red mist all over the concerned office inhabitants.
The mist is blood, sprayed out by the man's rupturing veins. Then the bike courier gets a telltale nosebleed, and he attempts to get out of the building by dies in the same painful way before he can get out. Walter makes the snap decision to quarantine the building, which traps Olivia and Peter inside.
So everyone but Olivia and Peter have died gruesome deaths, and Peter slips in some blood and is likely infected too, but everyone knows incubation rates are much slower for people whose names come first in the credits. A little detective work determines the man who died first was there to sell something to the oil company, and a briefcase found in the man's rental car contains a sample of the virus found in a geological sample.
Walter, butting heads with the suits at the CDC (who are preparing a "level 6 eradication" for the building), figures the virus is an ancient disease that wiped out mammals in the ice age, and the virus has enough of a personality that it wants to get out of the building, meaning the infected turn into rageaholics trying to smash their way out so the virus can wipe out the planet, oh, let's say 28 weeks later.
And yep, Peter's infected, but he manages to foil the tox screen prepared by his dad, and is almost out of the building before a nosebleed gives him away. Peter mad! Peter smash!
Walter, having gone inside to do the tox screen and not wanting to leave his son, manages to MacGyver up a sample of an antidote from the company's break room fridge, but the nearest chemical procurement facility or whatever that could make enough is too far away for the CDC's exterminator, and he's ready to send in the troops. Broyles and Olivia try to buy time by pumping a sedative through the building's ventilation system, but it was turned off to prevent the pathogen from escaping, so Olivia's got to head back inside to turn it on.
Inside, she's attacked by a fury-blinded Peter, who beats on her, grabs her gun and heads up the elevator so he can shoot his way out (not that he'd make it with the firepower the Army's bringing). But Olivia manages to get the ventilation system turned out, the gas is pumped in, everyone goes night-night for a while, giving Walter enough time to cure them.
And this should be noted: during the crisis, Walter, upset over Peter being infected, says, "I can't let Peter die again." Later, Astrid asks him what he meant. He tells her to mind her own beeswax.
In Boston, a bike courier rides up to an office building. We can tell he's a courier as opposed to a biking enthusiast because he's doing the pant-leg-rolled-up thing. He locks his bike up and heads into the building, past the Vitas Petrol sign.
Into the elevator he goes, which already holds an older man with close-cropped hair, who's already pushed the button for the 16th floor, which is where the courier's going. "Going to be a nice one today," says the courier as he glances over the man, receiving a curt agreement in accented English from the other dude. There's a little bit of sweat rolling down the older man's face. "You don't look so good," says the courier, which is easy for him to say, with his youth and his hairline! The older guy turns his head, and there's a little trickle of blood coming from his left nostril. He wipes his nose with a tissue, but there's more blood coming, and as they get off the elevator, he's pale and glistening with sweat. He wanders away from the direction of Vitas Petrol's reception desk, and the courier tells "Helen" that he thinks there's something wrong with the guy. Helen calls to him to tell him he needs to sign in, and the older guy looks at her, and it's not clear if he's even comprehending her.
Then he collapses and is on his back on the floor while people freak out and the courier performs CPR on the guy to no avail. He checks for a pulse around the guy's neck. "He's dead," the courier tells the office worker who was trying to help.
But wait, there's more! Suddenly the veins in the dead guy's body are all standing out in sharp relief as his body convulses, and a red mist spews from the corpse's mouth, splattering on everybody nearby, which is a lot of people, since the looky-loos had all come over to see what was going on. In a talking head, Michael Scott says, "Reminds me of the time Meredith collapsed after a bender the night before."
After the opening credits, we watch Astrid anxiously make her way through the Boston Children's Science Center, until she's stopped by a helpful employee. Astrid says she's looking for someone who's lost, and the employee assumes she's talking about a kid, but it doesn't take a genius to see where this is going. "Ah. A special needs individual," says the employee when Astrid tells him it's not a student but a man. "You have no idea," says Astrid.
Sure enough, there's Walter, lecturing a bunch of kids about the history of the first circumnavigation of the world, and that when the Victoria arrived back home, only eighteen of the original 237 crew members were on board. A little girl asks what happened to everyone. "They all died, little girl, horrible and most likely painful deaths," says Walter, looking slightly annoyed at being asked, while the woman with the "Science is fun!" sticker on her sweater looks uncomfortable. Heh. Walter starts telling them about the price to pay for opening certain doors, and gives them the example of how when you check under your beds for monsters, you are immediately eaten, whereas if you hadn't looked, you wouldn't wind up "being slowly digested in the stomach-sac of the creature." I'm not convinced this is as self-evident as he seems to think it is. The bright side, though, is that maybe their screams will serve as stark reminders of the grim butcheries that await us under our beds.