The hunt for the source of the deadly drug begins in the upcoming episode 3, titled "A War Zone," of the fourth season of the American police procedural drama "Chicago P.D."

A drug called Fentanyl is found out to be linked to the multiple overdoses around the city. The intelligence division is assigned to hunt down the dealers of this dangerous drug. In the investigation process and start of looking at possible sources, the team discovers that there exists a series of illegal drug dealers within a college smuggling ring. They are suspecting that these dealers may have a direct link to the one who is on top of the chain.

Tay (Li Jun Li) and Burgess (Marina Squerciati), meanwhile, encounter a child covered in blood. The child later on spills out the dark and twisted secret about his home life with his parents. Elsewhere, Mouse's (Samuel Hunt) plan to reenlist in the army may cause some issues, and Platt (Amy Morton) reveals to Tay that she will no longer be working on her current position as she has been reassigned to her former post.

The promo trailer for "A War Zone" shows the team discussing about catching the source of the fatal drug that they have been searching for the longest time. The clip also shows the team getting through what looks like the drug lab.

In other news, Jon Seda's character Antonio is set to leave the show to move to another "Chicago" spin-off series, the freshman "Chicago Justice." Executive producer Michael Brandt shared on The Hollywood Reporter about how the transition would work.

Brandt explained, "Jon's character on P.D. had been through a lot personally. He's a single parent, he's got a couple kids, so it made sense for that character to want to take a job that was more settled and conducive to being a parent."

The all-new episode of "Chicago P.D." season 4 airs next Wednesday, Oct. 26, at 10 p.m. on NBC.