All three episodes followed a unified story line of an infection spreading across Chicago and the first responders dealing with the repercussions.

While in the basement rescuing trapped people Severide notices a panel on the wall which has the "BRT" letters on it.

Intelligence discovers that the original victim was a lab assistant at CCU who had been telling professors that there would be an apocalyptic-type event in Chicago.

Voight and Upton begin interviewing tenants of the apartment building who all collectively describe a man who appeared to be an exterminator.

CCU students reveal that they were working with the infection hoping to develop a treatment for it although their progress in this regard was destroyed in the fire.

One apartment tenant mentions a blue pickup truck with a grey circle which intelligence begins searching for.

Intelligence tracks down a blue pickup in the vicinity of the apartment building and the tag plate comes back to a car rental company.

Voight and Jay Halstead arrive at Chicago Med looking for Seldon who had been identified as the truck rentee but he is nowhere to be found.

have trouble maintaining order when civilians believe they know who caused the outbreak and want to take matters into their own hands.

The team finds his lab with many medical experimentation tools and a map with an area circled that includes a parade route.

Firehouse 51 arrives to assist with the parade only to find the route to have no spectators due to widespread fear from the infection.

Further investigation leads them to believe that Seldon is planning to release the infection at BRT headquarters where a board meeting is taking place.

In the boardroom Halstead finds the entire board taken hostage by Seldon and begins trying to deescalate the situation.

Upton, with a sniper rifle on the adjacent building, creates a distraction which allows Voight and the rest of the team to enter.

[1] Executive producer on all three series, Dick Wolf, and Chicago Fire showrunner, Derek Haas, wrote the story for all three episodes.

"[5] Vlada Gelman with TVLine said that the "crossover proved to be a monumental event for some of the Windy City's finest.