SFPD Detectives Terry English and Hildy Mulligan are quickly thrust into the middle of a brutal onslaught as two students open fire on a school bus in the middle of the city. San Francisco's finest must work quickly in order to put an end to one of the deadliest shootings in recent history.

Alfie Rentman, one of the shooters who killed 18 classmates on a school bus, is still free. He's now a cop killer as well after he shoots officer Walt Martin in the tunnels where they were searching for him. The ongoing search of the underground maze finds a room Rentman used. It's also the place where he left a clue for them to find him. With Lt. Koto recovering from his wounds, Terry is made lieutenant and put in charge of tracking Rentman. The boys parents insist that Alfie is a severe schizophrenic and often delusional. When the police do track him down, they find he has a hostage and is apparently prepared to die. Written by garykmcd

The lawyers representing Dustin Maker decide to bring in outside counsel and focus on Jamie Nelson, who has been teaching for the last 5 years. She hasn't been in court for years but finds the case very interesting. The investigation into the death of Inspector Walt Martin continue and the medical examiner concludes that the bullet that killed him did not come from either of Alfie's guns. He was also shot from a distance of only a few feet which seems to rule out friendly fire. Hildy investigates the shooting of an 18 year-old Chinese man, Victor Chan, in what appears to be a gang war over marijuana grow ops. They've arrested 13 year-old Jalil Thompson who was there but isn't talking. Hildy learns that she has a new partner and that her brother, Jimmy McCormick, will be joining the squad. Written by garykmcd

Although they've found the gun that killed detective Walt Martin, Hildy and Terry aren't sure that his partner, Kaleb Peat, was the actual killer. The medical examiner finds an anomaly and concludes Kaleb's death was murder, not suicide. Hildy is still investigating the murder of Victor Chan as no one believes 12 year-old Jalil Thompson was the killer. Raffi Veracruz from the gang and tactical unit is upset and worries that Hildy's investigation will negatively impact her investigation into Sugar Cascade. Jamie Nelson visits Dustin Maker in hospital and decides to take his case. Molk and Jimmy McCormick continue the investigation into Sarah Tran's disappearance but the case takes a sudden turn. Written by garykmcd

With evidence that Det. Kaleb Peat's death was a murder made to look like a suicide, Hildy and Terry come believe his death and that of Walt Martin may have been cop on cop killings. Terry doesn't tell Lt. Koto all that he knows about the case as he and Hildy set out to clear as many of their colleagues as possible. Hildy starts with her brother Jimmy, who isn't very cooperative and doesn't have an alibi. Alyssa drops into see Molk for no apparent reason. Suger tries to make peace with the Chinese gang and it leads to a jailhouse killing. Dustin Maker meanwhile is transferred to prison but surprises his lawyers with his response after being told the prosecution seek the death penalty. Written by garykmcd

Terry and Hildy find themselves investigating people within their own department. Jamie digs into Dustin's past in an effort to better understand his actions. Meanwhile, Suger searches for answers as someone close to him disappears, then makes a shocking discovery.

The truth about Navarro leads Terry and Hildy to Mexico, as an angry Suger prepares for war. Jamie's casework takes her to Pleasantville, and a particularly painful event from Dustin's childhood. Raffi's investigation into Suger takes an unexpected turn.

Junior reveals new details which may finally give Hildy and Terry the upper hand in their investigation, while the Trial of Dustin Maker begins with opening statements. Meanwhile, an unlikely source provides Suger with startling new information regarding his sister.

Terry and Hildy's investigation is dealt a lethal setback, forcing them to close their case. Raffi's loyalty comes into question, while a cautious Suger contemplates the future of Potrero. The stress of the trial takes a dramatic toll on Jamie's personal life.

Witnesses take the stand in the case against Dustin. Meanwhile, Jamie faces fallout from her tirade, and threats are made against Hildy's family.

Terry and Hildy take a high-ranking officer into custody, while Suger faces a new and unexpected threat to Potrero. Meanwhile, Molk uncovers unsettling information about a potential prostitution ring.

In the season finale, Terry and Hildy are gloves-off as they race to close in on a murder suspect, while Fatty B lobbies to take Potrero in a new direction. The Trial of Dustin Maker reaches a climactic verdict.