Chicago PD focuses on the Chicago Police Department. It's a spinoff of Chicago Fire. Starting way back in the first season on Chicago Fire, the seeds were being planted for a new group of first responders to join their firefighting brothers and sister.
The force is headed up by Hank Voight (Jason Beghe) who started out as a less than desirable crime fighter on the sister series. He redeemed himself and now heads up the Intelligence Unit. Alongside him are Det. Antonio Dawson (Jon Seda), Gabriela Dawson's brother, former CI Det. Erin Lindsay (Sophia Bush), Det. Jay Halstead (Jesse Lee Sofer), Det. Adam Ruzek (Patrick John Flueger), Det. Alvin Olinsky (Elias Koteas), new Det. Kevin Atwater (Laryoce Hawkins), Officer Kim Burgess (Maria Squerciati) and Sgt. Trudy Platt (Amy Morton). As their foil we find Internal Affairs agent Edwin Stillwell (Ian Bohen) who both wants to bring Voight down for what he's done in the past and wants to join him to reap the rewards of skimming money on the job.
Throughout the series and in the wide open world of the NBC Chicago franchise characters from Chicago Fire come and go making the city feel as vibrant as it is.