Sixteen years later, Casey is a doctor in residency at a Chicago hospital. A mysterious flu-like illness is overwhelming emergency rooms. In the break room, Casey’s fiancé, Nate Warner, another doctor, kisses her. Nurse Garcia reports the ER is full. Nate orders beds set up in a hallway to handle overflow patients, prioritizing repeat cases to understand the worsening illness. Nate mentions his parents are visiting and asks to meet Casey’s father. Casey lies, agreeing to ask him, though she has not spoken to Dale in nine years.

Casey and Nate treat hallway patients. Casey examines Ms. Klein, who awakens disoriented with complete memory loss. When Casey orders a CT scan to rule out head injury, she discovers Ms. Klein’s fever has dropped to an impossible 89.8 degrees. A nearby male patient with a goatee suddenly lunges and bites Casey’s arm.

Chaos erupts as repeat patients turn feral, attacking staff and other patients. A security guard’s gunshots fail to stop them. Casey tells Nate they must flee, saying the attackers are no longer their patients. As they escape through the panicked waiting room, Casey realizes her father “was right about everything” (17). The infected burst from the hallway, and Casey and Nate fight their way toward the exit.

Six weeks after the outbreak, Casey and Nate hide in his Lincoln Park apartment with dwindling supplies. Casey watches a confused woman stumble down the street, being used as bait. Casey has classified the infected: Nomes lose all memory, biters become violent, and burners are surviving humans who have become lawless predators. She survived a bite, healing with only a scar, proving her immunity.

A pack of biters kills the Nome. Nate insists they leave for Dale’s compound in Wisconsin, but Casey resists, arguing the journey is too dangerous. She drunkenly revealed her past to Nate during isolation, and he will not drop the subject. While Nate cooks rice, it boils over; he drops the pot, then cuts himself and yells in pain. Casey worries about attracting biters or burners. Someone knocks on the door.

A man outside pretends to need help. Knowing it is a trap, Casey grabs a baseball bat while Nate takes a cast-iron pan. They open the door, and two burners restrain Nate, taking the pan and hitting him with it. A third burner confronts Casey, but she strikes him in the head with the bat, dislodging his eye. Another burner tackles and chokes her.

Recalling Dale’s self-defense training, Casey gouges the attacker’s eyes and breaks his nose. As she struggles to free herself from a third burner, Nate flees the apartment, abandoning her. Casey kicks the burner she blinded in the mouth and throws boiling water and rice in the last attacker’s face. She grabs her go-bag and retrieves a combat knife Dale gave her for college. Casey escapes to the alley and discovers the garage door open and Nate’s Porsche gone.

Casey heads to her old truck that Dale gave her as a teenager. She navigates the derelict Chicago streets, encountering a biter that smells the blood on her clothes. She kills it with her combat knife, stabbing through its brain stem. The truck struggles to start as a horde approaches, but the engine finally turns over. Casey drives through the biters, slamming on the brakes to throw one off her hood.

She plans a route using back roads to avoid gridlocked highways. Passing a group of burners who attempt to stop her, she speeds past them. After driving through the chaotic suburbs—with burning homes and bodies littering the streets—she reaches the relative calm of the countryside. Casey crosses into Wisconsin, reflecting on her vow never to return home. She acknowledges that the apocalypse has made the compound her only possible option.

Casey arrives at Dale’s property at night and parks down the road. She observes the barbed-wire-topped perimeter fence with dead biters tangled in it. Removing her engagement ring, she follows a memorized sequence of steps to find a fake rock containing the gate’s spare key.

As she walks up the driveway, someone holds her at gunpoint from behind. Casey swiftly disarms him, disassembling his pistol, and recognizes her cousin JJ. He is overjoyed she is alive, explaining Dale searched for her for weeks and was devastated, believing she had died. JJ leads Casey through the woods to the main house, where Dale sits on the porch. When he sees her, Dale is overcome with emotion, his voice cracking as he says her name.

Dale embraces Casey tightly. After JJ resumes patrol, Dale asks how she made it. Casey shows him the combat knife, and Dale says he “hate[s] to say [he] told [her] so” (44). When Dale asks if anyone is with her, Casey says no, concealing that her fiancé Nate abandoned her.

Dale leads her inside the house, now furnished to accommodate multiple families including relatives, neighbors, and people groups they met after the apocalypse. Dale gives Casey food and water, insisting he will always look after her. He takes her upstairs to her old bedroom, mentioning she will have a roommate. Casey enters and sees a muscular man asleep in the other bed. When he rolls over, she recognizes his green eyes. The man is Blake Morrison, her childhood bully.

Casey angrily confronts Dale about Blake being in her room. Blake, wearing only boxer briefs and displaying a very muscular physique, sits up and pretends not to recognize her. Casey recalls Blake filling her locker with canned goods and mocking her as she fell. In the present, she tells Dale that Blake was supposed to be left outside during the apocalypse. Dale tells them to solve things on their own and leaves.

Blake reveals he and Dale have become good friends, which infuriates Casey. He teases her about her hygiene, and she storms off to shower, leaving the bedroom lights on to annoy him. While undressed in the bathroom, Blake knocks, gets no answer, and opens the door a crack to hand her a towel. He notices bruises on her neck and shoulders and asks if she is okay. Casey dismisses his concern and slams the door. Looking in the mirror, Casey recalls Blake putting gum in her hair at school. She reflects that Blake “fooled [her] once” (51) before and vows it will not happen again.