A highly dysfunctional blended family is forced together when they realize a reported virus is actually the onset of the undead apocalypse.
Nick Clark wakes up in a heroin den at an abandoned church. He investigates a scream and finds his friend Gloria eating a person. Terrified, he flees outside to a pre-apocalyptic Los Angeles and is hit by a car.
At the hospital, a doctor informs Madison and Travis that Nick -- who has a bruised rib and contusions – was ranting and delusional when he was brought in.
Cops question Nick in his hospital room, but he stonewalls them. They ask why he was talking about blood and viscera, but Madison arrives and orders them to leave. Afterwards, she asks Nick to go back to rehab. He dismisses her concern.
Outside Nick’s room, Travis takes a call from Liza, his ex-wife, who puts him on the phone with their son Christopher. Christopher refuses to spend the weekend with Travis if it means being at the hospital with Nick. Travis urges him to treat Nick as part of their new extended family.
Travis offers to stay with Nick while Madison goes to school where she works as a college counselor. En route, Alicia tells Madison that Nick is getting worse and they can’t just send him to rehab like they did when he was a minor.
At school, a student, Tobias, sets off the security metal detector and is stopped by the principal, Artie. Madison covers for Tobias by frisking his pockets and pretending that loose change triggered the detector.
In her office, Madison asks Tobias why he tried to bring a knife to school. He reveals his fears about news reports of a virus spreading around the country. Madison assures him they’re safe and confiscates the blade.
Alicia skips class to hang out with her boyfriend, Matt, at the football field.
Nick tells Travis about the gruesome scene at the church and worries he might be insane. Travis speculates that the heroin gave Nick a bad trip, but goes to the church anyway to see for himself. He runs into a shell-shocked junkie and asks what happened. “Don’t kill me!” the man screams and runs off. Afterwards, Travis finds maggot-infested blood and guts all over the church floor.
The next day, Travis tells Madison what he saw and offers that Nick might not be crazy. Madison attributes the blood to everyday crime and asks Travis not to enable her son.
At school, Alicia cuddles with Matt, who suggests they meet at the beach for sunset.
Madison stops by Artie’s office and finds him at a switchboard, listening in on classes for teacher evaluations. “Nature always wins,” they hear Travis tell his students while teaching Jack London’s short story, “To Build a Fire.”
Back at the hospital, Nick convinces a nurse to untie one of his hands so he can use the bedpan. Suddenly, the patient in the adjacent bed codes, distracting the nurse long enough for Nick to escape. Before he sneaks out, he witnesses the doctors fail to resuscitate the patient and then quickly evacuate him to the morgue.
After learning that Nick escaped, Madison asks Travis to bring her to the church “where it all started.” There, she sees the bloodbath and agrees that something bad happened. She finds Nick’s book in the disheveled drug den — with a heroin needle inside. Travis comforts her as she cries.
Madison and Travis visit Nick’s high school friend, Calvin, to see if he’s heard from Nick. Calvin offers to call some friends for possible leads. Travis calls Christopher but it goes to voicemail.
Nick buys a disposable cell phone and leaves a message for someone. He rests under an overpass.
Alicia waits for Matt at the beach. “Where r u?” she texts. “You better be dead.”
Madison and Travis encounter a traffic jam on a highway off-ramp. As police cars and choppers swarm the area, a round of gunshots and screams erupt. Travis drives away from the scene.
The next day, Madison and Travis join a group of teachers at school who are watching footage of the incident that caused the traffic jam: As an EMT worker hunches over a victim, the victim rises up and ravages his neck. The undead victim proceeds to attack everyone around him, resistant to police gunfire.
Nick wanders down a street and leaves another message, begging the person to call him back.
In class, Alicia and her friends watch more footage of the incident. Alicia again texts Matt, “Where r u??” Over the P.A., the principal makes an unexpected announcement that school is ending early. Madison tells Alicia to go home in case Nick shows up.
As students flood onto school buses, Madison and Tobias share a knowing look.
Nick meets with his dealer at a downtown diner. It’s Calvin. Nick asks if his drugs were laced with anything that might have caused hallucinations. Calvin denies it and tells Nick that his parents came looking for him. He yells at Nick for putting him in danger of being discovered as a drug dealer.
Calvin drives Nick to the L.A. River and retrieves a gun from the trunk. Nick sees the gun and tackles Calvin, shooting and killing him in the process. He flees the scene.
Madison and Travis pick up Nick soon after. Nick tells them that he killed Calvin and takes them to the culvert. To his astonishment, Calvin’s body is gone. As they drive off, however, they see Calvin staggering toward them. Madison gets out of the truck to help Calvin but he attacks her. Nick runs Calvin over twice with the truck. As they gape at Calvin’s crumpled body, Calvin starts rasping and turns his mangled head toward them. “What the hell is happening?” Madison asks.