Firehouse 51 responded to a horrific warehouse fire that killed 39 people. This led the Chicago P.D. on a manhunt for the suspect. Now with someone in custody for both arson and multiple murders, it is up to Peter Stone and his team, including lead investigator Antonio Dawson to step in to bring some peace and resolution to so many families and friends of those lost - including one very close to home.

The crossover starts on Chicago Fire S05E15 Deathtrap (I) and Chicago P.D. S04E016 Emotional Proximity (II).

Stone and the State's Attorney team investigates the chain of events following the arrest and death of a young man, ultimately seeking charges against Police, Fire Department and Medical personnel, only to have a recalcitrant Grand Jury indict a single individual: Officer Kevin Atwater. Paul Robinette defends Atwater.

When a Muslim grad student is brutally murdered, Stone and the team arrest his friend, another Muslim student, who claims he committed murder to prevent a terrorist attack. The jury must decide whether he's a killer or a hero.

When a prominent judge is shot and killed while leaving a bar with Valdez, her integrity is called into question and Stone prohibits her from working the case. The team learns the judge's death is connected to a trial where he handed down a lenient sentence to a college rapist.

When a man is found stabbed to death in his home, Nagel and Antonio discover he was an ex-Navy SEAL who was writing a book that exposed the truth about a top-secret mission. Stone becomes convinced that the SEAL was killed to suppress the real story of what actually happened on the failed mission.

When a pregnant woman is found murdered and her baby cut out of her, the State's Attorney's Investigators race to find the baby and the killer. The case is complicated when it's revealed that the killer's DNA matches that of her father: a convicted serial killer already in prison. Both killers then attempt to use the DNA evidence as their ticket to freedom.

When a female juror on Stone's case against a gangbanger winds up dead, the team believes she was murdered because of her guilty vote. They later learn that she committed suicide - driven to it by the cyber harassment of her former boyfriend. Stone goes after him for murder.

When Abigail Chapman returns from Spain after being acquitted for murdering her boyfriend while studying abroad, she finds herself the prime suspect in another murder case involving a young college student. During the trial of the real killer, the Second Amendment is put to the test.

After an innocent girl is killed in a gang-related shooting that was instigated on social media, Stone's obsession for justice endangers the family of a cooperating witness. To salvage his crumbling case, Stone must go head-to-head in the courtroom against both the social media providers who inadvertently fueled the gang war and a cocky young man who may have pulled the trigger.

When an anti-environmentalist Alderman is killed during a vicious hit-and-run, Antonio and Nagel go all out to track down the driver. But instead of this being an organized hit to save the environment, it turns out the driver is a frantic mother desperate to save her kidnapped daughter. When Valdez discovers the mother was victim of a scam, the legal question becomes complicated: who is responsible for the Alderman's death? The mother trying to save her daughter? Or the perpetrators of the scam?

After a man is shot multiple times and left for dead, and a kilo of diluted heroin is found in his car, Antonio discovers other similar cases where murder victims were found with the same type of adulterated drugs. Ultimately, the narcotics detectives involved in the case-one a decorated hero-appear to be the common thread. Meanwhile, Stone is hesitant to let Nagel be involved in the case, as she has a close connection to the victim.

A crane collapses at a construction site and kills the son-in-law of wealthy real estate developer, Frank Linden. Nagel and Antonio, with the help of and CFD Chief Boden discover that the collapse wasn't actually an accident. The team digs into Linden, who has made many enemies, more dangerous friends, and is accustomed to using his money to get his way.