A man gets off an elevator in a Boston high-rise. His nose begins to bleed and within moments of stepping onto the floor, he passes out. A bike courier begins to give him CPR when suddenly his veins begin to bulge through his skin, followed by him spewing out blood.
Olivia and Peter have started the investigation when the rest of the team arrives. While Peter and Olivia are upstairs questioning witnesses, Walter and Broyles stand outside and happen to witness the bike courier dying in the front lobby. They soon realize that what caused this man to die is a deadly virus. It is contagious and looking for a host to live in.
They quarantine the building. The four city blocks surrounding it are shut down, and the CDC is brought in. Walter tries to solve the case from outside the building. However, Olivia and Peter are still inside and possibly infected. They try to keep the witnesses calm. As time goes by, the virus keeps contaminating people and causing them to react violently. The virus eventually kills its victims.
Walter and a team of CDC agents enter in hazmat suits in order to weed out the non-infected people. Once Olivia and Peter are cleared, they begin to exit the building. Suddenly, the CDC agents realize that Peter is infected. He is locked in the building with the other infected victims.
Knowing that the life of Peter and the other victims are at stake, Walter finally discovers the antidote to the virus that is made up of household items. They still need to figure out how to calm the already-infected. Walter figures out a solution to kill the virus, but they still need to get close to the infected people to cure them. In their sickness, the infected will kill anyone to protect the life of the virus. As a last resort, Olivia goes back in to the building to activate the ventilation system so that fentanyl gas can circulate and knock the victims unconscious. This helps them administer the antidote. The team succeeds and Peter's life is spared.