Dawson tries to strike a balance with Casey during her first day as a candidate. Mills ponders an uncertain future. Severide faces his demons. A city inspector visits Molly's.

Casey deals with the aftereffects of his sister's divorce announcement. Dawson and Mills get used to their new roles at Station 51. Brett ponders whether to leave Chicago. The Molly's crew works to get the food truck started.

Casey faces his corrupt brother-in-law and Brett goes above and beyond to help a mother in need. Meanwhile, Mills get an education on unlicensed private investigation and Dawson begins training with Herrmann.

An apartment fire connected to a case of rape and attempted murder resembles an unsolved case in New York, so Sgt. Benson of Special Victims Unit comes to Chicago to help Intelligence with the investigation.

The crossover continues on Chicago P.D. S02E20 The Number of Rats (II) and Law & Order: Special Victims Unit S16E20 Daydream Believer (III).

Boden's job is on the line pending an internal investigation. Severide adjusts to Patterson, his new commanding officer. Dawson shakes things up at the work. Casey searches for important evidence in his investigation of Nesbitt. Herrmann tries to keep the peace with Molly's new neighbors.

Evidence that seems to exonerate Boden may not be enough. Brett and Chili find themselves in a precarious situation. The team responds to a stuck elevator in a hotel. Jimmy Borrelli's older brother wants him transferred to a different house.

Boden offers to host a wedding at the firehouse after a fire destroys the original location, but it doesn't sit well with Chief Riddle, who also butts heads with Dawson over her ability to perform. Severide fights for his old position. Capt. Patterson finds himself torn between leadership and his colleagues.

Boden's new neighbor asks him for a favor that could have serious repercussions. Severide is upset when he's treated like a candidate by Patterson. Cruz is visited by a gangbanger who knows his brother. The team responds to a call that involves an 18-wheeler; and is thanked with tickets to see Rush.

Boden suspects his neighbor is setting him up; Severide, who's still on probation, receives a surprise visit from his father and also learns something about Patterson that could be a game changer. Romance blossoms at a fire department gala.

Boden believes Maddox is behind Serena's accusations. Severide reaches out to Jamie, who's Maddox's lawyer, in an attempt to help. Patterson questions whether Boden can remain chief. Brett and Chili suspect that a man passed out in a garden is connected to a previous call. Otis has problems with his social calendar.

Patterson fills in for Boden, who's trying to clear his name. Severide learns where Serena may be hiding. Herrmann and Otis give Freddie a job that keeps him close by. Jimmy and Chili draw more attention then either bargained for.

Freddie's father begs Herrmann to forgive his son's assault. Casey discovers that a makeshift shelter housing people displaced by the tornado may close. Casey attends a fund-raiser at the request of an alderman. Dawson and Brett experience repercussions after telling Boden about Chili's erratic behavior.

The team responds to a mass shooting; Casey investigates the missing money from the fund-raiser. Jimmy agrees to represent the firehouse in a boxing match against the Chicago PD's 21st District. Chili's behavior continues to be an issue for Brett.

Severide confronts Chili about her erratic behavior. Members of the firehouse rally around Casey, who they think would make a great candidate for alderman. Jimmy squares off against Antonio in the first annual "Battle of the Badges" boxing match.

The team responds to a suspicious call from a neighboring firehouse; and welcomes a new firefighter, though her addition has repercussions. Casey moves forward in his run for alderman.

Severide and Cruz find a child's remains inside a chimney while investigating a possible carbon monoxide leak inside a home. In other events, Casey braces for the worst in his run for alderman while Alderman Becks continues to bad mouth him to the press.

The team responds to a building collapse that's trapped more than a dozen people. Dawson's quest to foster Louie is aided by a familiar face. Otis, who's on medical leave, meets his temporary replacement. Severide and Kidd continue to see each other. Casey is at odds with Antonio.

Boden and Jimmy are at odds. Kidd deals with her unstable ex. Dawson continues her quest to foster Louie. Casey's new political consultant urges him to think big. The team responds to a dangerous structure fire.

Severide and Stella search for Grant, who's AWOL from Chicago Med. Dawson questions her career path. Casey's relationship with his political consultant cools. Borrelli's anger at Boden over his brother's death boils over. Mouch and Brett partner on a new venture.

Casey and Dawson find themselves in the midst a potential political mess. Stella has to make a big decision about her ex-beau. The squad responds to an auto accident. Severide befriends a socialite. Brett and Mouch set aside their writing differences for the greater good.

Casey seeks help with a personal matter from Susan Weller. A rich party guy wants Severide to travel with him. Stella tries to help a sick street kid. Otis and Herrmann share their concerns about Molly's lack of customers.

Lt. Casey is unable to rescue a woman who becomes trapped inside her burning greystone, prompting him and Lt. Severide to butt heads over the husband's role in the tragedy. Herrmann is on a mission to catch a mystery tagger that keeps targeting their firehouse and Boden gets a surprise visitor. Meanwhile, Dawson and Brett respond to a 16-year-old boy who was injured during a romantic homecoming proposal.

While responding to a call in the field, Dawson accidentally hits a pedestrian who stepped in front of the ambulance, and is left to deal with the repercussions. Lt. Casey and Lt. Severide disagree and continue to investigate an arson case while Chief Boden is left to decide whether to attend an important function in New York. Elsewhere, in hopes of preparing him for a possible promotion, Lt. Casey puts Herrmann at the helm for a shift so he can discover firsthand what his superiors deal with.

Dawson and Casey deal with legal matters in hopes of remaining Louie's legal guardians. Severide continues in his efforts for a bone marrow procedure to help Anna, but when things don't go as planned, he finds himself slipping back into old habits. Meanwhile, Brett opens up to Dawson and members of Firehouse 51 become competitive, much to the dismay of Chief Boden.

The crossover continues on Chicago P.D. S04E09 Don't Bury This Case (II).

The team is on high alert when a previous incident during a fire call leaves a dangerous man on the loose with a vendetta to target Lt. Casey and Firehouse 51, and prompts CPD to step in. Dennis Mack, a buddy firefighter of Severide's father, comes to town to shadow the squad but may have ulterior motives. Meanwhile, Brett and Antonio's relationship hits some bumps in the road when his ex-wife Laura surfaces and Casey has a present for Dawson.

When the squad's rig breaks down in the field, Casey reunites with his old friend and colleague Jason Kannell from Squad 6. Dawson and Brett serve their time in retraining. Severide tries to remedy what is going on with Anna and finds support from unlikely sources. As Connie returns from her family engagement, the crew lays bets on how long her temporary replacement Marcy will last.

Cruz finds himself in a bad spot when an off-duty incident begins to unravel, prompting Mouch to step in and offer help in hopes of avoiding serious consequences from the CFD. Casey goes head to head with a big politician to get his measure for first responders passed. Lt. Severide makes life difficult for newest team member Jason Kannell and Dawson gets an unexpected visit from her father.

Lt. Casey gets off on the wrong foot with Sam Mullins, a temporary Chief who is filling in for Boden at House 51 for the morning. When Casey later makes a risky move in an attempt to save a distraught man from jumping off a building, Mullins has a surprise that Casey does not expect. Mouch flies in Cruz's brother Leon as a birthday surprise, but the rest of the squad struggles to keep it a secret. Meanwhile, Brett is caught off guard when she learns new information about her hometown friend Hope.

After heroically saving a mother and newborn baby from a rooftop fire, Kidd receives surprising news when she discovers she is being transferred out of the firehouse. Severide and Casey believe something is off about the turn of events, prompting them to investigate the situation. Otis and Herrmann discuss the possibilities of taking over a rundown bar and Dawson's frustration with her father, Ramon, reaches a boiling point.

After saving a young girl, Dawson is forced to make a tough decision when she finds out the girl's father is addicted to painkillers. Herrmann loses his cool and gets into an altercation with a police officer after responding to a call. Otis and Cruz are on a mission to find out who Brett is dating. Meanwhile, Boden is skeptical when Donna's brother Julian unexpectedly comes to town.

Dawson and Casey do everything they can to locate Bria's whereabouts after discovering important new information. Lt. Severide reunites with Chief Grissom, and old family friend. Meanwhile, Cruz enlists the help of Brett and Mouch to create a new tool and Kidd and Zach go on a date.

After responding to a time-sensitive rescue, Casey and Severide don't see eye to eye on the best course of action. Herrmann struggles to entertain his daughter on "bring your child to work day." Brett is joined by a surprise guest during a spa getaway, courtesy of Dawson, while Kidd's social life gets a boost.

After responding to a car accident, Casey and Dawson become skeptical when a domineering husband continues to answer for his injured wife. Cruz becomes frustrated with Herrmann and Mouch when he discovers they have contacted people regarding his "Slamigan" invention without his knowledge. Kidd uncovers something meaningful and important to Severide while Brett discovers some potentially unsettling news.

In response to a call involving a residential fire, Otis and Kidd's lives are suddenly put in jeopardy when gunfire goes off inside the house. Dawson comforts Brett during an especially trying time. Meanwhile, Casey begins to notice that romance might be going on within the firehouse.

Boden has a lapse in judgment in the aftermath of rescuing a young boy. Kidd struggles to win the approval of Severide's mom after a surprise visit and Otis takes on new responsibilities at the firehouse.

When the FBI takes over Firehouse 51 for an undercover mission, Casey and Severide volunteer to assist with the investigation. Kidd attempts to help Severide come to terms with recent developments he's been managing.

After responding to a residential fire at a property belonging to a Mexican drug cartel, Casey is skeptical of Cordova after a hefty amount of money was found missing from the crime scene. Elsewhere, Kidd makes a big decision.

After saving a girl from a car accident, Chief Boden and Lt. Severide are taken aback when they receive a surprise package. Brett struggles to keep quiet about Cruz reacquainting with his now-married high school sweetheart and Otis tackles his next challenge. A crisis brings Bria back into Dawson's life.