Still reeling from the death of informant Anna, Voight faces a new threat as he tries to keep the neighborhood clean. New wrinkles develop in the Upton, Halstead and Voight dynamic. The team gets a new chief.

The team investigates a string of brutal pharmacy robberies, and Halstead connects with one of the victims about their Army careers. Upton suspects there is more to the story and confides in Voight.

Upton's dogged pursuit of a missing teenage girl leads the team into a dark web of human trafficking. Chief O'Neal becomes involved with the unit's investigation when it becomes evident his son, Sean, has a connection to the missing girl.

Atwater’s reactions during a tense arrest are called into question. The team must dig in to uncover key evidence that could aid in the ongoing investigation and clear Atwater’s name.

When actionable evidence finally surfaces against Sean O'Neal, Upton and the team work relentlessly to build a case in secret. They discover Sean is more elusive and dangerous than anticipated.

To distract Intelligence from the investigation into his son, Chief O'Neal forces the team to handle a "dead fish" case. However, when that dead fish turns into a complicated heater case, Burgess is thrown back into difficult memories from her past.

After a shocking discovery, the team begins closing in on Sean O’Neal with every shred of evidence they can find. Determined to keep his son out of prison, Chief O’Neal hires a lawyer.