Before “Chicago P.D.” Season 4 premieres on NBC, it will be helpful to jog out memories and remember what happened in the explosive Season 3 finale. Episode 23 began with Hank Voight and his team investigating the death of a single mother -- 26-year-old Melissa Wilds was found in her own car’s trunk with a slit throat.

It takes a while, but Voight (Jason Beghe) immediately connects Melissa’s death to his son Justin who suspiciously left town recently to “help a friend.” But Voight is proved wrong when Justin turns up on the brink of death the same way Melissa did – with a slit throat in his own car’s trunk. It doesn’t take long for the sergeant to slip to the dark side completely. He becomes angry, thirsty for revenge and doesn’t hesitate to do anything in his power to find his son’s killer.

Meanwhile, Commander Crowley (Barbara Eve Harris) offers him the position of lieutenant and he has 48 hours to either take it or reject it.

Hank does every questionable thing humanly possible to get information on who killed Justin. Crowley doesn’t approve of his tactics, of course, but they are effective.

In the “Chicago P.D.” Season 3 finale, Voight finally visits his brain-dead son in the hospital and it’s clear he has no chance of survival. Now all that is left to do is pull the plug. However, before doing that, Voight bids a teary farewell to his son. The scene proved to be a huge tearjerker.

Finally, Beghe’s character learned that the man who killed his son is none other than a person named Kevin (Joseph Sikora). He meets Kevin and coaxes him to confess murdering Justin. The already heated situation becomes worse when Keith offers half a million dollars to Voight in order to make it right.

In the end, Hank kills Keith, but not before his adopted daughter Lindsay (Sophia Bush) arrives on scene to persuade him not to cross the line. It doesn’t work, and she flees the scene of crime.

What will happen in the Season 4 premiere of “Chicago P.D.”? The sergeant will have to face the consequences of his actions and it’s clear in the promo that Crowley suspects he did something really bad. She pressurizes Lindsay to spill the beans.

Watch the “Chicago P.D.” Season 4 premiere on Wednesday, Sept. 21 at 10 p.m. EDT on NBC.