Chicago P.D. uncovered one of the team member's serious problem with addiction, leading to a tense winter finale.
In the beginning of "Descent," Antonio (Jon Seda) was first seen at a doctor's opposite and he rejects a call from Ruzek (Patrick John Flueger). The doctor arrives and tells Antonio that his injury seems to have healed properly so there is no reason for him to keep experiencing pain.
Antonio, who looks rougher than usual, asks for a new refill on his painkiller prescription, setting red flags for the doctor. The doctor says no, saying he could lose his license if he gave a refill when it's not indicated. Antonio leaves in a rage.
He then returns home and desperately looks for more pills, punching his bathroom mirror in a moment of frustration. The shockers come later, however, when the team raids a drug house and arrest a big time dealer. Burgess (Marina Squerciati) notices a man rushes out of the house and pursues him, and when she is about to shoot the man, he turns around and it turns out to be Antonio.
The reveal leaves the team feeling suspicious, despite Antonio's excuse that he was there following a clue from a confidential informant. He is later shown to have gotten drugs from the dealer. The next day, Antonio walks into the jail and tells the dealer he did not mean for him to get arrested that night. It is revealed that Antonio had agreed to protect him from police if he got him pills for free.
Desperate for a fix, Antonio reaches out to a new dealer who sells him heroin instead of pills. But before Antonio gets to his car, Voight (Jason Beghe) intercepts him and confronts him on his drug problems.
"I'm sorry I don't know what the hell happened... the dealer, Carter. I know him," Antonio admits. "He was giving me pills."
Voight takes Antonio home with the intention to take him to detox later but things escalate when Antonio gets a call and it's revealed that Eva has been kidnapped.
The situation leads Antonio to come clean about his addiction problems to the rest of the team, who all work together to find Eva before it's too late. The investigation leads them to the dealer's cousin — things get worse when it's revealed that Eva's captor had been previously convicted with statutory rape.
As the team follows the man's trail they find his apartment. Antonio trashes his room searching for clues of his daughter's whereabouts and finds a bottle of pills, which he hides before Jay (Jesse Lee Soffer) fetches him. Shortly after the team finds another home associated to the man and they finally find Eva, who was badly attacked and possibly raped during the traumatic experience.
With Eva on her way to Med, the team follows the man's trail to an abandoned factory that drug addicts frequent. Voight and Ruzek apprehend the man, and as they are about to make the arrest, Antonio shows up and starts to beat the man up. Voight and Ruzek let him get a few punches in before separating them, but the man then provokes Antonio talking about how he abused his daughter.
In the moment of anger, Antonio pushes the man to what turns out to be a fake wall, and he falls to his death. Ruzek begins to panic wondering how they can spin the situation to not get in trouble. Voight starts to think and asks Antonio if he is sober, he admits he took some pills earlier that day so he would fail a drug test.
The episode ends as Voight looks puzzled wondering what to do next as police sirens approach and the man's dead body is seen lying on the ground below.