We begin in the 1960s with a scientist on the news saying that fungus can take over if certain things in the world change, like the temperature.

As everyone looks at him like he's crazy, we cut to 2003.

Sarah wakes up and has breakfast with her father. Tommy arrives, and he and Joel are going somewhere.

The neighbor asks Sarah when she's going over and Joel says after school. He then offers them biscuits but Joel says he's doing the Atkins diet and Sarah will them all about it after school.

At school, Sarah sees twitches in someone next to her. She goes to the store to get the watch fixed but the store owner's wife tells him they're shutting the store.

Sarah returns to the neighbor's house and bakes cookies. She finds a movie she thinks her father would like and returns home.

They watch the movie and she falls asleep. When she wakes up, Joel has left to get Tommy out of prison.

Sarah goes back to the neighbors but they've started turning into the infected. Joel and Tommy arrive in the nick of time and the trio leave together in the truck.

They're caught up in a plane crash and everyone else trying to leave town. They all decide to go on foot, but Joel is chased by the infected and a solider kills the infected.

However, he is then given orders to kill them both and shoots Sarah. Tommy kills him.

We flash to 2023 and a young boy is walking in Boston and faints. He is taken into a Fedra area and tested. He's infected. The woman tells him they'll get him food but she injects him and Joel is one of the people to burn his body.

Joel is tired of the way things are and wants some clarity. He wants to find his brother and Tess wants to help him do that but they got swindled for their battery.

Marlene is the leader of the fireflies and she has Ellie locked up. 

She tells her she has a higher purpose and that it's time for them to think about it.

Ellie says the fireflies are awful and Marlene counters that Ellie had a friend in the fireflies.

This hits a nerve.

Joel and Tess go looking for their battery and find the man dead and Marlene injured.

Marlene asks Joel and Tess to transport the girl because she's now unable to and she promises they will get a vehicle and weapons in return.

They reluctantly agree and take Ellie back to Joel's. Ellie checks out the radio and the book and realizes there are messages being sent.

At night, they leave and embark on the mission, but they are stopped and checked for infection. Ellie is infected, but she claims she's been so for 3 weeks and something is different with her.

Bill sends an '80s song through the radio as they escape the zone.