Begin with the inaugural season of “Chicago Fire,” which can be watched without interruption. However, it’s important to note that Episode 23, “Let Her Go,” serves as a backdoor pilot for “Chicago P.D.,” marking the first official crossover of the One Chicago universe. “Chicago P.D.” launched alongside “Chicago Fire’s” second season, allowing viewers to alternate between episodes of both series up to Season 2, Episode 20 of “Chicago Fire,” titled “A Dark Day.” This episode’s storyline seamlessly transitions into “Chicago P.D.” with the episode “8:30 P.M.”

The crossover events continue in “Chicago Fire” Season 3, Episode 7, “Nobody Touches Anything,” which features characters from “Chicago P.D.” Additionally, the stories interweave with “Chicago Fire” Season 3, Episode 13 (“Three Bells”) and “Chicago P.D.” Season 2, Episode 13 (“A Little Devil Complex”), focusing on the tragic death of Leslie Shay (Lauren German). The complexity escalates as both series partake in crossovers with “Law & Order: Special Victims Unit.” Episodes to watch include “Chicago P.D.” Season 2, Episode 20 (“The Number of Rats”), “Chicago Fire” Season 3, Episode 21 (“We Called Her Jellybean”), and “SVU” Season 16, Episode 20 (“Daydream Believer”), as the teams tackle linked arson cases. The introduction of “Chicago Med” adds another layer to the One Chicago tapestry.

The first season of “Chicago Med” premiered simultaneously with “Chicago Fire” Season 4 and “Chicago P.D.” Season 3. Its backdoor pilot aired as Episode 19 of “Chicago Fire.” Continue alternating episodes from these series until reaching “Chicago P.D.” Season 3, Episode 21, where the fire department becomes embroiled in a self-contained crime episode on “P.D.” Following this, “The Beating Heart” and “Malignant” are notable crossover episodes, appearing as “Chicago Fire” Season 4, Episode 10 and “Chicago Med” Season 1, Episode 5, respectively.

The first season of “Chicago Med” debuted at the same time as Season 4 of “Chicago Fire” and Season 3 of “Chicago P.D.,” with the show’s backdoor pilot airing as Episode 19 of “Chicago Fire” that year. Continue alternating episodes until you reach Season 3, Episode 21 of “P.D.” The entire fire crew is involved in a crime that is self-contained on “P.D.” Next comes “The Beating Heart” and “Malignant,” which are Season 4, Episode 10 for “Chicago Fire” and Season 1, Episode 5 for “Med.” 

In the crossover, Christopher Hermann (David Eigenberg) ends up getting stabbed, inspiring all three teams of professionals to race against the clock to save his life. During Season 3, Episode 10 of “Chicago P.D.” (“Now I’m God”), the force consults with the staff at Chicago Gaffney when a friend of Hank Voight’s (Jason Beghe) stands accused of intentionally overdosing his cancer patients on chemotherapy drugs. On top of that, the backdoor pilot for “Chicago Justice” aired as the 21st episode of “Chicago P.D.” that season.

From there, you can cruise along steadily until Season 5 of “Chicago Fire.” Here, a three-way series event between “Fire,” P.D.,” and a freshly-debuting “Chicago Justice” unfolds. For “Fire,” the episode in question is the season’s 15th, “Deathtrap,” and for “P.D” is its “Emotional Proximity” — Season 4, Episode 16, with the first episode of “Justice,” “Fake,” rounding things out. The crossover concerns a club fire that ultimately takes the life of Alvin Olinsky’s (Elias Koteas) daughter, Lexi (Alina Taber). Characters from “Med” appear on the “Fire” and “P.D.” episode, adding a fourth show into the mix. “P.D.” and “Fire” share a crossover that happens during Episode 16 of “Chicago Fire” and Episode 13 of “P.D.” that year, featuring Gabriella Dawson (Monica Raymund) and Sylvie Brett (Kara Killmer) going undercover to nab a bad guy. From there, the order simplifies — but if you want the whole story, you’ll have some “Law and Order” episodes to watch.

Once “Chicago Justice” is out of the way, it’s simply a matter of weaving in the rest of the crossover outings in the order in which they aired until you reach the current season. As of this writing, that includes a high-rise fire that gets personal and spans Season 7, Episode 2 of “Chicago Fire” (“Going to War”), Season 4, Episode 2 of “Chicago Med” (“When to Let Go”), and Season 6, Episode 2 of “Chicago P.D.” (“Endings”). A fight breaks out between departments involving Cruz (Joe Minoso), Hank Voight (Jason Beghe) and an arson case which takes up Season 7, Episode 15 of “Chicago Fire” and Season 6, Episode 15 of “P.D.” Then there’s the infamous “Infection” arc, which spans Season 8, Episode 4 of “Fire,” Season 5, Episode 4 of “Med,” and Season 7, Episode 4 of “P.D.”, as Chicago battles a deadly bacteria.

“Fire” and “P.D.” have another crossover, in which Severide and Voight deal with a crime connected to the opioid crisis, in Season 8, Episode 15 of “Fire” and Season 7, Episode 15 of “P.D.” Finally, there’s the TV movie event which marks One Chicago’s post-COVID return to the shared universe game, “In The Trenches.” Dubbed one of the best One Chicago crossover events, it sees a multilayered disaster grip the city, forcing all three teams to work together, in Season 13, Episode 11 of “Fire,” Season 10, Episode 11 of “Med” and Season 12, Episode 11 of “P.D.”

But the crossovers don’t end there. If the “Law and Order: SVU” episode mentioned above intrigued you at all, you might want to follow the two meetings “P.D”  had with the “Law and Order” franchise in Season 15, Episode 15 of “SVU” and Season 1, Episode 6 of “P.D.” A second crossover happens in Season 17, Episode 14 of “SVU” and Season 3, Episode 14 of “P.D.” You won’t need to watch all of “SVU” to understand what’s going on, but seeing both episodes will definitely enhance your understanding of “Chicago P.D.” — and the One Chicago world at large.