This year's One Chicago crossover felt like The Walking Dead, delivering a gripping three-hour event filled with drama, suspense, and one hell of a case. Things begin peacefully enough, with the crews from Chicago Fire, Chicago Med, and Chicago P.D. tailgating ahead of a Chicago Bears game. Their fun is cut short when Casey (Jesse Spencer) sees a guy collapse and Dr. Manning (Torrey DeVitto) discovers a severe infection on his leg. With no ID and no one around to claim him, Will Halstead (Nick Gehlfuss) and the rest of the Chicago Med staff have no idea who this guy is or how he contracted the flesh-eating bacteria currently chomping away at his limb.

Coincidentally, Ambulance 61 receives a call about a couple who haven't checked their mail in days. After Foster (Annie Ilonzeh) kicks the door open without even breaking a sweat, she and Brett (Kara Killmer) find the pair deceased and covered in wounds similar to the John Doe's. It looks like this flesh-eating bacteria, which has a four in a million chance of being infectious, is spreading quickly. But how? Even Jay Halstead's (Jesse Lee Soffer) lead on Patient Zero, who turns out to be a college student named Stuart, comes up short. Stuart traveled to Botswana while serving in the Peace Corps, but his trip was so long ago that it's impossible for the bacteria to remain dormant for that length of time and suddenly become active. Clearly, they're dealing with something new... and deadly.

Taylor Kinney, <em>Chicago Fire</em>Taylor Kinney, Chicago Fire

More calls start coming in, including one where Burgess (Marina Squerciati) finds a woman and her infant child also infected. Their deaths become the top news story, as media outlets begin reporting about the outbreak, which has now claimed five victims. Firehouse 51's no good, very bad day becomes exponentially worse when they're alerted to a fire at Chicago University. A research lab with a billion dollars worth of equipment has gone up in flames. It's a close call, with Truck 81 battling green flames and a stubborn door that refuses to open, but they're able to get everyone out. Unfortunately, the lab specialized in researching different diseases, which means the crew may have been exposed to something deadly. They're hosed down by hazmat workers and Stella rightfully freaks out. Leave it to Severide (Taylor Kinney) to ease her fears by discussing their future together.

"It's not how I'm gonna go," he says when Stella asks how he thinks he'll die. "[I'm gonna die] an old man in bed, with our kids and grandkids, and you surrounding me." That's sweet, except for the part about Stella watching him pass away.

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You know it's serious business when Boden (Eamonn Walker) uses his stern voice — the one powerful enough to wake the dead — to call in Chicago P.D.'s Sgt. Voight (Jason Beghe). A quick pow wow with both leaders, plus Casey and Severide, reveals the facts: The sprinkler systems were cut and the lab was paid for by BRT (which Patient Zero kept chanting). Severide thinks the fire was arson and that it's tied to the bacterial outbreak wreaking havoc on the city.

Meanwhile, Ruzek (Patrick John Flueger) and Jay look into Patient Zero, aka Stuart, whose condition is worsening at Chicago Med. They grabbed files from the fire, but Jay insists Ruzek put on gloves before touching them. (This episode was sponsored by Lysol and Purell.) They unlock Stuart's cell phone and uncover threatening messages on the university servers about an apocalyptic event, changing Stuart from victim to suspect. Burgess and Upton (Tracy Spiridakos) rush to the hospital to interrogate him but Goodwin (S. Epatha Merkerson) reveals that he died, along with their lead.

Elsewhere, Severide concludes that the lab fire was intentional, like he suspected. It turns out the lab was researching antidotes for necrotizing fasciitis (say that three times fast), which can be caused by numerous bacteria. Upton goes to check on a woman from the lab who worked closely with Patient Zero and didn't show up for work the day of the fire, even though she was scheduled to. She, too, has been infected and collapses in Upton's arms, covering the detective with her blood in the process. At the hospital, Manning finds a cut on Upton's finger which means she might have been exposed to the bacteria as well.

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As fear spreads across the city, Bret and Foster, along with several other paramedics, get radio calls of multiple cases at an apartment complex. The seizing victims have all been exposed to the same bacterial infection as the previous five, which seems to refute Goodwin's insistence that it can only be spread through open wounds. Reporters are in a frenzy and no one knows how this thing is spreading. It's bad.

Nick Gehlfuss, <em>Chicago Med</em>Nick Gehlfuss, Chicago Med

An influx of 30 new patients forces Goodwin and Maggie (Marlyne Barrett) to set up a quarantine in an old wing of the hospital. But Maggie, who is currently going through chemotherapy for metastatic cancer, is sent home as a precaution. Meanwhile, Chicago P.D. gets closer to the truth thanks to a good Samaritan who informs Platt (Amy Morton) that an exterminator was spraying the apartment building which has become ground zero for the outbreak. This rules out Stuart as a suspect since all cases have either come from that complex or the lab.

Will teams up with CDC worker Andrea Danner, who he went to school with, on a possible antidote. After commiserating over their romantic woes, they realize the bacteria is spreading at alarming rates, which makes it difficult to stop. The bad news keeps rolling in with the city breaking out in a panic, incensed by the 24-hour news cycle that has taken Goodwin's words out of context. The quarantined patients have grown restless and break out of the hospital, but they are quickly rounded up in police vans. Plus, Marcel (Dominic Rains) is working around the clock on patients as another one codes, and Firehouse 51 is called in to control a mob who have pounced on a Salvadorian woman's restaurant based off of a right-wing conspiracy website wrongfully accusing her of being behind the bio-attack. It's getting pretty hairy in the Windy City.

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In some much-needed good news, Upton is cleared to return to work just as Burgess and Ruzek circle in on their new suspect, who turns out to be one of the lab workers. Yeah, it's the same guy currently working on an antidote at Chicago Med alongside Will and the CDC. Will continues his streak of terrible luck (because getting placed into Witness Protection on his wedding day and watching his girl fall for Ezra from Pretty Little Liars isn't bad enough), and walks in on said lab worker destroying their samples. During a struggle, the suspect knocks Will out cold with a microscope. Jay finds his brother, who is miraculously not dead and somehow able to describe what happened despite the quarts of blood draining from his head. I think Will Halstead might be a superhero.

The suspect, Dr. David Seldon, has escaped and Chicago P.D. embarks on a manhunt to find him. Halstead and Upton track down his wife who, after a violent standoff in a panic room, reveals that Seldon isn't done with his attack on the city. Upset that his research on infectious diseases was turned down for funding by BRT, he set out to prove them wrong by inciting his own apocalypse. But a guy that smart isn't going down without a fight and Seldon plants numerous leads, sending the police on a wild goose chase. Chaos nearly claims the life of a Spanish-speaking man who was almost shot to death by a terrified patrol officer who thought he was the suspected terrorist.

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Intelligence eventually tracks down Seldon's secret lab and realize his next target is the Octoberfest parade. However, CFD and Chicago police roll out with guns blazing to an empty street. The city-wide panic kept everyone at home, leaving the area eerily quiet. Intelligence later figures out that his final act will be getting his revenge on the BRT board members who shut down his research. Firehouse 51 and Intelligence arrive at BRT's offices, where Seldon is holding hostages. Seldon has infected board members with the deadly bacteria and given them a choice to either die by a bullet, or fund his research and live.

Jay tries to talk him down by volunteering to help prove him right. Who knew that meant handing Jay a knife to cut himself and then spray the open wound with infected water? It looks like Jay is actually thinking about doing this, not knowing that Will and Andrea developed an antidote. Is he mad?! It's a good thing Voight ordered Upton to distract Seldon long enough for him to bust through the door and shoot Seldon dead. Who else is having trouble breathing after all that?

With the hectic situation winding down, Ruzek invites Burgess to come home with him because, yes, we can have nice things sometimes. As if we couldn't be more in our feelings, Stellaride end the night passionately kissing, and Will and Jay wrap the episode in a bar together, taking in the lively night life. Cheers, indeed!

One Chicago airs Wednesdays on NBC starting with Chicago Med at 8/7c, followed by Chicago Fire at 8/7c and concluding with Chicago P.D. at 10/9c.

Marina Squerciati and Patrick John Flueger, <em>Chicago P.D.</em>Marina Squerciati and Patrick John Flueger, Chicago P.D.