Halstead and Upton are getting hot and heavy when they're interrupted by a phone call from the hospital notifying her that her father suffered a heart attack.
As they drive over to Med, they divert to respond to a call about a shooting.
The dead man is identified as Mike Blaine, a cop with 22 years on the job, who took two bullets to the back.
Voight knew Blaine as they worked together at VICE for four years.
He informs Intelligence that they need to work the case by the books since so many eyes will be watching them.
Blaine was murdered off-duty in a rough neighborhood. The tire marks at the scene of the crime indicate a Prius was involved.
There are also texts to a burner phone, which indicates he met up with Marco Perez, a shot caller for the Latin Jokers, who Blaine previously arrested.
Perez informs Intelligence that after he got out of jail, he reconnected with Blaine and they became friends.
He met up with him that day to loan him some money. He also recalls a white man at the scene in a Prius. Intelligence locates Zach, a graphic designer, who was arrested by Blaine at a recent protest.
He began calling for justice on an anti-police website and made online threats against Blaine.
They bring him in for questioning, but Zach doesn't want to talk. Eventually, he admits that he was stalking Blaine to prove he was a bad cop since his complaints against him were ignored by the Chicago police.
But the one thing Zach didn't do? Kill Blaine.
So, who did?
Platt notifies Upton and Halstead that someone spotted a car dumping a gun in the alley. The alley belongs to a Hispanic woman named Maria.
They arrive at Maria's house, but she makes a run for it. When they eventually bring her in for questioning, she says she would never kill Mike because he saved her life.
However, her gun matched the gun that killed Mike.
She explains that Mike intervened in a domestic violence situation and promised to protect her from her violent ex-boyfriend, Jose, who stole her gun a few days prior.
Maria admits that Blaine gave her the three grand and told her to lie low in a hotel while he talked to Jose.
They look into Jose and learn he was two blocks away from the crime scene.
Upton and Halstead pay Zach a visit hoping that maybe he can place Jose at the scene.
A black Audi pulls up and begins shooting. Upton protects Zach. After speaking out on the news, Zach is now a witness and a target.
He still refuses to speak because his hatred for cops is greater than doing the right thing.
Intelligence finds the stolen Audi and matches the blood inside to Jose.
They locate his car, but their plan to get him goes south when an innocent bystander gets dragged into the picture.
Eventually, Halstead is able to disarm Jose. Ruzek finds video footage of Luis dumping the gun.
Voight pays Superintendent Deputy Samantha Miller a visit to make a case for Blaine, who he believes died a hero.
She hears him out and eventually honors Blaine in a press conference.
Upton agrees to go see her father before his bypass surgery, but when she gets to the hospital, she can't bring herself to speak to him.
That evening, she tells Halstead that she was glad she got to talk to him.