'Chicago P.D.' season 4 spoilers, plot news: Showrunner teases Lindsay and Halstead's origins

The fourth season of NBC's "Chicago P.D." will delve more into the past of Erin Lindsay (Sophia Bush) and her lover Jay Halstead (Jesse Lee Soffer). The couple is said to be going through a rough patch as Lindsay will try to protect Hank Voight (Jason Beghe) after he killed his son's murderer. She will have to do this while saving herself from being charged with accessory to the murder.

Season 4 will reportedly have Lindsay leaning on to Halstead while the issue with Voight is still hot. The showrunner of "Chicago P.D.," Matt Olmstead, said that this season will look more into the respective histories of the two lead characters this season, and it will hopefully give the audience a better understanding of what's going through the character's heads.

In an interview with TV Line, Olmstead said: "There won't be a designated storyline where they're [Halstead and Lindsay are] off on a road trip ... we want to dig a little bit more into [their respective backstories] as it relates to both of them. That's a focus for us this year."

A bit of Lindsay's past was actually introduced to the show already. Viewers have already met Erin Lindsay's mother, and reports say that this season will have her father coming into the picture. With the words of Olmstead, it may seem that Lindsay's parents may be a prominent fixture in the upcoming season of "Chicago P.D."

The season premiere will have Commander Crowley (Barbara Eve Harris) suspecting that Voight and Lindsay are hiding their involvement in Justin's case. She believes that Voight did kill his son's murderer, but she lacks the evidence to prove it. Lindsay will have to choose between protecting her mentor or saving herself from becoming a part of an investigation.

The fourth season of "Chicago P.D." is scheduled to air starting on Sept. 21 on NBC.