The episode kicks off with Intelligence investigating the murder of newly-elected Mayor Kelton.
Voight becomes a prime suspect since he was spotted cruising around the house around the time of the murder.
The sergeant ignores the accusations and continues on with the case.
First, they track down a gang member from the Southside Hustlers that was texting Kelton using a burner phone.
They find the guy responsible for the text threats, Monroe, but he says the texts were about the money Kelton failed to pay up.
Turns out, he was using the local gangs to canvas for him.
They pursue a second lead, but the gang member reveals that his gun was stolen by a cop who raided his place.
Halstead recalls the very raid he's talking about and recalls Voight being a part of it.
All the evidence against Voight is staking up.
Halstead is approached by the lead homicide cop who tells him that they're pursuing Voight.
When Halstead refuses to turn on his boss and help them build a case, he threatens to ruin his career when they bring Voight down.
Halstead regroups with Atwater, Burgess, and Upton, and they all decide to stand by Voight regardless of whether he's guilty or not.
With all of their jobs on the line, Halstead confronts Voight about his involvement.
Voight tells him he wasn't forthcoming about where he was the night of the murder because he's protecting Antonio.
As Ruzek tracks down Antonio's car, he runs into his sponsor who tells him the cop was in pretty bad shape.
Voight confirms that he took him to an off-the-books center to get the necessary help.
Halstead feels like an ass for thinking his boss was capable of murder.
They look into the other officers who were part of the raid and find Baker, a cop known for stealing from crime sites.
Upon arrival at Baker's apartment, they find out that he passed away a few weeks ago and his belongings were cleared out by Kate Brennan.
They find footage of Brennan, who claimed she was at a party drunk till the wee hours of the next morning, arriving at Kelton's house just before he was shot to death.
She tries to deny it but eventually cracks to Voight.
She tells him she was doing Chicago a favor and he doesn't disagree.
He leaves her house alone, which doesn't sit well with Halstead. Since Voight had all the evidence, he should have arrested Brennan on the spot.
As Voight drives away, Halstead enters the house and finds Brennan in her cop uniform and wielding a gun.
When she sees him, she puts it up to her jaw in an attempt to kill herself.
Halstead and Upton wrestle the gun away from Brennan and arrest her.
When they bring her to the precinct, she's furious with Voight thinking he betrayed her.
Voight is furious with Halstead since he disobeyed orders and didn't trust Voight's plan.