The plague that's engulfed the city of Philadelphia has claimed Kate as its latest victim. With the help of the CDC's Charlie Stafford, Megan is able to stabilize her friend. Kate is quarantined as the rest of team works to determine what the ten or so people who weren't part of a cluster of victims have in common. Dani happens to be one of the outliers in question.
Since anyone who is infected dies within 72 hours, FBI Special Agent in Charge Brendan Johnson suggests that one of the victims may also be the terrorist who spread the disease. Megan visits Kate at the hospital where she learns that guy Dani met at the bar is still alive even though it's past the 72-hour mark. This man could be the terrorist. Megan has a different theory. Perhaps one of the 17 drugs he's on is keeping him alive.
Peter has been desperately trying to account for Dani's whereabouts the week before she was killed. He's also been desperately trying to get hold of her mom who is out of the country. It's been a difficult time. During an interview with a reporter, he calls the terrorist a loser and a coward. Later, Peter goes off on Megan when he feels she's being insensitive to his grief. In truth, she just can't put a stop to what's happening without him.
Megan and Charlie realize that the killer is taking meds to stay alive so that he can infect more people. They are able to identify the disease they are dealing with as virus in the Ebola class. The good news is that it's not actually Ebola. It's very rare and they should be able to determine lab from where the terrorist got it. The bad news is that there is no cure.
Megan knows the CDC must be working on a cure for this thing. She gets Charlie to admit that there's an experimental drug that's in the works. Megan wants him to use it on Kate, who is grateful to see a friendly face when Curtis visits her. As for the experimental drug, Charlie breaks the law by using it on a patient. It's not Kate. It's the guy Dani met at the bar. He's their best bet since he's remained alive the longest... until now. The guy has a fatal reaction to the vaccine. It seems there really is no cure. That means Kate is almost out of time.
Bud, Sam and Peter realize the terrorist typically spreads the disease one location at a time. Peter realizes that one of those locations was a mobile one-like a bus. Security footage shows all of the victims from the list of outliers boarding the same bus. They all touched the railing that had been infected by the killer a few hours earlier. They identify him a disgruntled former worker at a lab that stores bio-hazardous strains.
Megan realizes that two of the outliers had bedbug bites that may have come from the same hotel where the killer stayed. They raid the place. The terrorist is gone, but likely not far. Sam, Bud and Peter realize he's headed to a subway entrance to infect the railings. They corner him on the stairway. When the man charges, Peter shoots him dead in the head.
The terrorist is gone, but the terror he spread lives on in his still-dying victims. Megan realizes that the first recipient of the experimental vaccine died because there was alcohol in his system when he received it. That means the drug could still work. Kate is living proof that it does. She wakes up surrounded by all of her colleagues, surrounded by all of her friends.