Chicago P.D.: A New Romance, A "Seismic Shift" and 5 Other Shocking Season 4 Twists

All eyes will be on Hank Voight (Jason Beghe) when Chicago P.D.returns for Season 4.

Voight's revenge killing of his son's murderer in the Season 3 finale will weigh heavily on his colleagues, particularly Lindsay (Sophia Bush) -- who, let's not forget, also lost what was essentially her brother.

"The main source of conflict is going to be between Lindsay and Voight," showrunner Matt Olmstead tells TVGuide.com. "She obviously tried to dissuade him from taking the law into his own hands, and he was dead set on going forward. So, what we have is her trying to come to terms with what she is pretty sure he did."

As for the rest of the team, "the unit itself is torn, as any cop unit in any city would be, if their commanding officer's son was murdered," according to Olmstead. "It's a family. You feel for him, you put yourself in his shoes. ... They were sealed off intentionally by Voight. They were sent to the other side of town on a goose chase, and they realize that ... in a way it was for their own benefit, so they wouldn't have to know anything or see anything."

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Lindsay's tangential involvement makes her a target of Commander Crowley (Barbara Eve Harris) early on, which could put her career in jeopardy.

"[Crowley] sees kind of a weak link in Lindsay, and goes to her and makes an appeal to her based on a couple things. 'Do you agree with what happened, or what we think happened? When are you going to think for yourself? Have you been fully absorbed into Voight's world?'" Olmstead explains. "And through this journey in the first episode, it's Lindsay really taking some body shots, in terms of, what's the right thing that I should do, and is it the right time to maybe distance myself a little bit from Voight?"

Dealing with that question proves tougher than expected, as Lindsay has to keep her mouth shut about what she may or may not have witnessed Voight do. "She can't tell anybody, because to lean on someone is to have to tell them information that would be incriminating. So she's trying to protect [the team] by not talking about it," Olmstead says. "Halstead's in a tough spot, because he wants to be there for her, yet she's withholding information from him. ... His advice to her is, 'Don't go down the tubes with Voight,' making her head spin a little bit even more than it already is."

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1. A turning point for Lindsay and Halstead

Lindsay's dilemma over what she may or may not have witnessed with Voight also carries over into her relationship with Halstead (Jesse Lee Soffer). "For so long, she's tried to keep that a secret out of respect for Voight as a boss and as a father figure, and this event really kind of wakes her up," Olmstead says. "There's a little bit of a shift in her life of, I'm going to start spending that emotional currency and loyalty on Halstead as I move forward as a woman, as opposed to worrying about my dad."

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But that's not necessarily a bad thing -- Linstead's step forward will include the pair (finally!) cohabitating. "[Halstead] sees what she's going through. He sees all the turmoil of this case and says to her, 'Move in with me,'" Olmstead teases. "Which is kind of a lifeline for her, in regards to what she's going through. And again, because of what she does and doesn't do regarding Voight ... she emerges from it ready to commit and put time into this relationship. So we are planning on going forward with the mechanics and the ups and downs of these two moving in together."

2. A "seismic shift" in Lindsay and Voight's relationship

It's no exaggeration to say that Lindsay and Voight's relationship will never be the same after the events of last season. "She has a huge debt towards Voight, who saved her life. Going forward, it's more [her] spending time with Halstead and less with worrying about Voight, which is kind of heartbreaking for Voight," Olmstead says. "When you're the mentor and the mentee finally leaves, you're proud because you've raised and prepared a student, but then it's a little sad when the student does leave. But there comes a time when they do have to leave. And this is what we're playing right now, this kind of seismic shift in the relationship between Lindsay, Voight, and by extension, Halstead. Voight does articulate to Lindsay, 'You're all I have left.' And he's carrying that, too, seeing someone kind of leave the nest who's also the last loved one [he has] out there."

3. A new partner for Burgess

Quanticoalum Li Jun Li is joining the cast as Julie Tay, who will replace Roman (Brian Geraghty) as Burgess' (Marina Squerciati) partner. The two get along "like a house on fire," according to Olmstead, but Tay -- who was recruited to the precinct by Platt (Amy Morton) after the two crossed paths at an event for women in law enforcement -- has some skeletons in her closet.

Tay's back story was inspired by the story of an actual cop that was relayed to Olmstead. She joins the unit from a previous detail that found her handing out parking tickets in front of the courthouse, "which is generally acknowledged as the worst job for patrolmen in the city, because it's usually [writing tickets for] cops who are there to testify," Olmstead explains.

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The reason she was stuck on that duty? "Tay says that she was working a regular job as a patrolman and there was a commanding officer who ... tried to plant one on her," Olmstead says. "She pushed him off, and as a result, he, in the parlance of Chicago P.D. , put a brick on her. That's why she's been writing tickets. As the story was related to me, which was really frustrating to hear, when a younger female officer either dates or spurns the advancements of a commanding officer, they're really at the mercy of what the guy can do or not do. And, so she's been penalized for it. And in her mind, she just hopes that she can stay at the 21st ... because this guy really is on the warpath. So that's her immediate concern, is getting out from underneath this guy."

She'll look to Burgess to help her deal with that conflict in the first three or four episodes, with the two getting along "like a house on fire," according to Olmstead.

4. A blast from the past for Lindsay

The Voight drama won't be the only challenge Lindsay has to deal with in the first part of the season, when she starts getting mysterious phone calls and flower deliveries at the precinct. Is it a "local wackjob" playing stalker, or someone who has a deeper connection to Lindsay? Let's go with the latter. "Someone from Lindsay's past, a long, long time ago, resurfaces," Olmstead says. "We've played a lot with Bunny, her mom. This is someone from the past as well."

5. A near-deadly encounter for Platt

Platt has a major storyline coming up this season when she and her father get jumped around Episode 5. The sergeant is "beaten to almost within an inch of her life," Olmstead says, and her father is in even worse shape. In the aftermath, it's "all hands on deck" to find out who attacked them. Sound familiar? "It's a little funhouse mirror version of what Voight did at the end of the season," Olmstead says. "His loved one was killed, and we saw what he did. And strange enough, now we see Platt, when she finds out what happened to her dad, kind of seeking vengeance, and Voight now trying to dissuade Platt from seeking vengeance, when he himself would be a hypocrite to try and dissuade her. ... It's a very shocking episode."

6. A new romance for Dawson

Look for yet another Chicago P.D.-Chicago Firecrossover couple when Antonio (Jon Seda) strikes up a romance with Sylvie Brett (Kara Killmer). Here's hoping we get to see a double date with them, Platt and Mouch (Christian Stolte).

7. A test of friendship

Mouse (Samuel Hunt) and Halstead butt heads when Mouse decides he wants to re-enlist, much to Halstead's dismay. And Lindsay finds herself caught in the middle of the two friends. "Halstead is adamant about him not going back over. Which has much more to do about Halstead than Mouse," Olmstead says. "It culminates in a really moving story about that relationship, that friendship between Halstead and Mouse, the loyalty, the promise that they would look after each other over here."

Chicago P.D. returns Wednesday at 10/9c on NBC.