At a 10K Zombie Run, a group of the "undead" stumble upon a dead body. They pose for photos next to it and post them on Facebook. When Jim, Carlos, Daniel, and Manus show up to investigate, the body is gone. Carlos gets a DNA sample from some decomposition fluid left behind and cadaver dogs find a pinky toe. Daniel is soon able to identify the body from a Facebook photo tag: it's Bonnie Williams, one of the organizers of the race.
Jim interviews Bonnie's roommate, investigative journalist Jessica Shelly, who says Bonnie hasn't been around lately because she's been busy with the race. She says Bonnie was quiet, loved zombies, and did odd jobs. Jim notices a framed article about Jessica hanging on the wall, where she is holding up a journalism award for her piece, "The New Low on Getting High." Jim finds and takes Bonnie's phone. Carlos finds a number of gag gifts in the apartment: packages of Zombie MREs. Jim determines most of Bonnie's zombie items came from Nick's Novelties.
When Jim and Carlos visit Nick Preston, the owner of Nick's Novelties, they quickly notice he is running a head shop, as there are bongs and drug paraphernalia everywhere. Jim suspects the Zombie MREs are bath salts, but because they say "Not for Human Consumption" on them, they technically aren't illegal, so he can't confiscate them. Carlos buys a package so he can investigate it at the lab. Nick says he hasn't seen Bonnie since earlier that week; she asked him to post a flyer about the race in his store.
Manus tells Jim about Denny Warren, a homeless man Bonnie befriended recently. But when he harassed and threatened her, cops brought him in for a 72 hour hold since he looked like he was emotionally disturbed. Bonnie thought he was high and hoped the police would force him into rehab, but his drug test came back clean, so they let him go. Jim looks for Denny at a nearby park he frequents and finds him growling, stumbling around, and looking like a zombie. He calls a squad car to get him and finds Bonnie's body in Denny's shopping cart.
At the hospital Callie takes care of Denny, who is handcuffed to a bed, still growling. Jim won't be able to talk to him for a while. Callie says bath salts don't show up on drug tests unless you're looking for them.
Daniel gives Jim a safety deposit box key he found on Bonnie's key ring, and Nick's financials reveal that he bought a lot ephedrine, a component to bath salts. It looks like he makes the bath salts too. Carlos' autopsy reveals that even though Bonnie wasn't a long time drug abuser, but she did die of a massive bath salts overdose. Her water bottle contained the same bath salts found in her system. Nick's Zombie MREs also have the same chemical composition.
Jim visits Nick with a warrant for his drug lab. Nick says what he's doing is not illegal and that he has a B.S. in Microbiology and is working on his Ph.D at Tampa Tech. Jim thinks Bonnie was dealing drugs for Nick and they had a falling out because she started rehabbing his customers; a good motive for murder.
At the hospital, Jim and Callie argue about September 7 as their wedding date. It's her cousin's wedding anniversary so she doesn't want it to be her wedding date, but Jim says that's a good day for his family to visit. Jim looks over Denny's clothes and finds a paper with Jessica's phone number on it.
In a homeless encampment, Jim finds Jessica in shabby clothes, trying to blend in. Jim says he read her award-winning article and that Jessica's "unnamed sources" sound like Nick and Denny. He thinks she would do anything to keep her drug addict source Denny high, which is why she had Zombie MREs in her apartment and why she has a great motive for killing Bonnie. Jessica denies it.
Daniel finds a USB and a blue ribbon in Bonnie's safety deposit box. Carlos says the FBI found that the Bonnie Williams in their database is a missing woman from Washington state and her DNA sample doesn't match what Carlos sent them. Bonnie Williams is not their victim.
Jeff confesses to Callie that he wrecked her car. He gives her $740 toward the repairs. He took it to take a girl he likes, Tina, to the movies and hit a post leaving a parking lot. Callie says she wants to meet the girl Jeff was willing to break the law for, but he doesn't want to introduce them.
Carlos is going to do another autopsy on the victim to find out more about how she lived. In the hospital, Denny, who is now coherent, denies killing Bonnie. He liked her because she was trying to help him. He starts throwing up blood.
At the hospital, Callie deals with a horde of patients who look like zombies — they are grey and stumbling around. Callie tells Jim there is an infection going around and they don't know what it is. The day before people were coming into the hospital with flu-like symptoms, but now their muscles are atrophied, their tongues are swollen, and they are throwing up blood. A lot of them were at the Zombie Run.
Manus reports that the FBI checked their database for people who matched the DNA sample of their victim and says her real name is Mary Baxter. She is also originally from Washington state. She disappeared 11 years ago, one year after Bonnie. Daniel found a query letter on the USB, a pitch to sell an article, "The Real Zombie Apocalypse" about the days leading up to a mysterious illness that puts hundreds of people in the hospital with flu-like symptoms that rapidly get worse. It was created by Jessica. Jim realizes it sounds exactly like what's happening at Well-Core Medical. Carlos hunches over, sick and looking like a zombie. He quarantines himself in his lab to find out what's making everyone sick and to find a cure. Jim thinks whoever killed Bonnie may be the same person who started this "zombie apocalypse." Carlos says it could be Jessica, since she predicted this would happen three weeks ago.
Jim approaches Jessica with the query letter and reveals Bonnie's real name was Mary Baxter. Jim suggests that Bonnie was hiding proof Jessica had prior knowledge of a criminal act and was going to turn her in. She knew people were going to be intentionally infected with a disease and did nothing about it. Jessica wasn't going to let Bonnie spoil the scoop of her career, so she killed her. Jessica says no, the article was just based on other incidents around the country.
Manus finds Denny was staying at a group home up until he was arrested, which got him kicked out. Denny had a motive to kill Bonnie since it was her fault he became homeless. But Denny denies killing her. He talked to her the day before the run at the park. He wanted her to help him get back into the home, but she wanted him to go to rehab. He didn't get high until after he left her, after scoring bath salts from Nick, who also came to the park. Denny figured he was there to hit on Bonnie again. He was obsessed with her, but she didn't like him.
Daniel shows Jim a flyer he got from Bonnie's old school district with some blue ribbon winners. There's a photo of the real Bonnie as a child, but none of Mary. Daniel finds a death certificate for Bonnie's mother, who just died the week prior.
Carlos, who is still sick, finds no evidence of infection, bacteria, or virus on Mary's body, but her hat was covered with spores from some kind of fungus. Someone put them there knowing she'd be around a lot of people. Nick shows up at the station, responding to a call from Jim, who played into his big ego by leaving him a voicemail asking for help stopping the infection. Nick complies.
Jim tells Nick that Carlos tried his vaccine and is much better; he is at the hospital distributing it. Nick thinks he saved the day, but since he created the virus in the first place, Jim's going to arrest him. Jim says he found out Nick doesn't have any advanced degrees. He was rejected by Tampa Tech three times — just like he was rejected by Bonnie. Nick used Bonnie to help cause an outbreak that only he could cure. He saved everyone, except for Bonnie, who overdosed on Nick's Zombie MREs. But Nick says he didn't kill Bonnie. The spores he used for the outbreak weren't lethal.
Daniel shows Jim a photo of the blue ribbon winners from Bonnie's high school yearbook, which also gives information on what they won for. Jim pretends to give Jessica a scoop on how Nick admitted to causing the outbreak, and says Nick admitted telling Jessica his plans three weeks ago so that she could have an exclusive story. Then Jim shows her the yearbook photos of herself, Bonnie, and Mary. They were all friends until Jessica had a falling out with Bonnie over a guy, who was her boyfriend first, but then dated Bonnie. Jessica was jealous of Bonnie because she took her boyfriend and won a ribbon for an essay, so Jessica killed her and took the ribbon, which is visible in the framed article hanging on Jessica's wall. But then Mary took the ribbon because she felt guilty knowing one of her best friends killed her other best friend, and she became Bonnie. Mary knew Bonnie's mother was dying of cancer and wanted Jessica to tell her the truth about her daughter's murder, but Jessica refused. Mary took the ribbon to give it back to Bonnie's mom. Jim arrests Jessica for the murders of Bonnie and Mary.
Callie tells Jim that the reason she doesn't want to get married on September 7 is because that's the day she met Ray. They agree to choose another date. Tina arrives at the house and her and Callie hit it off.