The department's intelligence unit hunts down vigilantes in the upcoming episode of "Chicago P.D." season 4.

Titled "Army of One," the new episode centers on a group that tortures and kills pedophiles. According to the official synopsis, the officers of Chicago P.D. come across a live video showing a tortured man being beaten and burned alive. Upon further investigation, Intelligence finds out that the victim has been previously arrested for statutory rape.

After a similar attack happens, the officers begin to suspect that there is a connection between the crimes. They also find a link between alleged pedophiles and a group known as the Prev Hunters. Elsewhere, Erin Lindsay (Sophia Bush) encounters some issues in her career and personal life.

NBC released a new promo for the episode and reveals that Hailey Upton (Tracy Spiridakos) accepted Hank Voight's (Jason Beghe) offer to be part of the team. She will be working on the new case with Lindsay, Voight, Jay Halstead (Jesse Lee Soffer), Kevin Atwater (LaRoyce Hawkins), Alvin Olinsky (Elias Koteas), and Adam Ruzek (Patrick John Flueger). The sneak peek shows the team watching the video of a man being burned alive, so they immediately rush to the scene of the crime. Children are involved in the case, which makes Lindsay even more determined to catch the suspects because she has a soft spot for kids. The teaser ends with the death of a young boy.

In the previous episode, District 21's Intelligence Unit took down some bank robbers who turned out to be teenage boys. It was later revealed that the suspects were being exploited by a partially paralyzed mastermind. After robbing thousands of cash from a bank, the group was eventually caught. The case was very difficult for Lindsay because she accidentally shot one of the teenage suspects in self-defense.

The fourth season of "Chicago P.D." airs every Wednesday at 10 p.m. EDT on NBC.