The Division investigates the murder of Sara Allen who was looking for her daughter. At the scene a note was found telling Sarah's daughter that she was a bad mom but was still looking for her daughter. Magda meets "" a floating inspector "" named Regan (who is filling in for Jinny who is on maternity leave) Magda doesn't like Regan and soon learns that the inspector has a lot of questions to answer. As Magda and Regan look into the murder of Sara Allen Regan is looking at footage of a news report and soon sees a suspect on screen. Kate asks Magda to go undercover at a foster care agency to look for the daughter of Sara Allen only to discover that several children are missing. Reagan does some investigating of her own while serving as a guard for Magda. She carries with her a picture of David (the person she saw on the news report)

Kate learns that a coma victim has awakened after nine years. Kate hopes to solve the case with the help of the victim. Also Regan learns that her son David was accused of stalking someone and is also schizophrenic. Now Reagan asks Kate if she can look at the files. Stacey Reynolds tries to help Emma Weber to reconnect with her family she is worried that her son won't remember her after nine years. Emma identifies the person who killed her husband Evan nine years ago from a picture in the newspaper.

Jinny has to figure out how to tell Jack she is in love with Hank. The Division tries to find out why two gay men were attacked at an AIDS Benefit. Kate tells Sally that Nick may helped attack the two men because his friends did but Sally wants to have the case tried as a hate crime. Kate tells Sally the two men were watching the concert peacefully. Kate talks with a suspect who remembers seeing to girls get angry because their parking spot was taken.

The investigators try to figure out how a small time drug dealer was killed and why. The division learns that Jennifer Davis knew Ray Turner when she was in Las Vegas. Magda tries to learn if Ray was ever abusive to her. Nate tries unsuccessfully to end an affair with Suzanne because he knows her husband is a cop. Stacey learns Mr. Hicks paid a lot of money to mess up a DNA test because he found out that Jennifer Davis isn't the biological daughter of Joe Davis.

Reagan continues to find evidence to prove her son's innocence. When Reagan and Kate try to interview him in prison they don't get much information out of David as to whether he killed Mary O' Reilly. Stacey is asked to help her friend track down his missing sisters. Stacey learns that the missing sisters came from Chechnya . After the sisters of Stacey's friend are located in a truck inside the truck four bodies are found dead due to dehydration. Stacey 's friend is asked to see if he is sister Katrina is one of the bodies in the truck. Miquel tells Magda that Katrina is still missing. Reagan goes to visit David who questions whether he killed Mary and wonders what might happen if he is convicted of the murder.

Raina tries to why D.J. a baseball player is accused of Rape, but Raina is almost taken off the case because Johnny (Raina's Boyfriend is a friend of the baseball player. Magda gets a surprise from Gabe a divorce. Now they must figure out to tell Ben.

The inspectors look into the mysterious kidnapping of a young lady named Melissa and wonder if the kidnapping is real. Raina questions Tom about the disappearance of his stepdaughter Melissa leaving Nate and Raina to wonder if Tom is telling the truth. Magda tries to figure out if she did anything wrong the night her husband returned home from a music tour. Jinny tries to get Magda to understand what she would do if the situation were reversed (if her husband had an affair) The disappearance of Melissa starts to get really strange when Melissa arrives at the San Francisco police department giving everyone there a big shock. The inspectors also learn that Melissa's father is a sexual predator.

The inspectors look into the murder of a pregnant woman and wonder if her husband could be connected in the murder. The inspectors learn that he was rock climbing alone at the time of the murder. Later, new suspects keep popping up and in a shocking ending that will leave you speechless, the murderer is revealed.

The inspectors try to figure out why a high school student was shot. The principal of her school doesn't believe that it is a serious problem. Later, a car is set on fire in the school parking lot. Jinny talks with Hank about having their wedding held at her father's home and trys to convince him to let Casey and his partner get married there as well. The detectives discover a cryptic website with horrific threats that will keep you in suspense until part two of this two parter.

Internal affairs looks into whether Jinny followed police procedure in an incident in which she shot the man who killed her brother (in the season 3 finale). Raina must testify about her part in the incident and is faced with a serious decision. Nate and Raina look into a suspect's hunger for sex possibly linked with a murder.

Jinny tells Kate that she is pregnant and is told to stay off the streets and work behind a desk Jinny is unhappy with the policy regarding police officers that are pregnant and wishes she could be on the streets until she goes on maternity leave. Now she has to tell hank that he is about to be a father. Jinny tells hank that he will be a father but as he learns this news from jinny Nate eavesdrops on the conversation between Jinny and Hank while he is waiting to ask a favor of Jinny . Nate asks Jinny to supervise a search while he goes and speaks with the District Attorney. Lisa learns that her husband is a rested for selling drugs and now must talk to the investigators and try to get her husband's charges reduced.

Stacy Newland helps Jinny & Magda figure out happened in a nightclub. The Medical Examiner tells Raina & Jinny something interesting about the body. The parents of Lee Anne are told she was she shot after believing she died in the nightclub fire. The Medical Examiner tells Jinny & Magda that their shooter was a man not a woman.

Kate, Raina, Nate, Magda, and Jinny must find a way to get Stacy, the principal, and 10 students out of the school safe and sound when they're held hostage by armed terrorists. The team meanwhile must figure out who these people are and why they're doing this. Also, Jinny and Hank get married.

Jinny & Magda are trying to find burglar Grant coal train but they must deal with a bounty hunter who is also looking for the burglar. The methods used by the bounty hunter drive Jinny & Magda crazy. Kate fights to keep her Granddaughter from the surrogate mom that wants to adopt her.

In this episode Nate gets Raina as a partner because CD dumped him. Jinny tells Magda that she got divorce papers from Jack. Raina tries to find out why CD left. Kate considers hiring Stacy Newland to help Kate. Amy whose husband hit her is killed by ""Buzz"" their pit bull was going to be trained as a guard dog. Jinny meets Hank Riley a sketch artist but doesn't seem ready for love.

Raina and the missing woman are trapped together in the small basement with no apparent way out. Kate, Nate, Stacy, and Magda meanwhile are shocked to learn the surprising identity of the kidnapper and are extremely worried when they learn that Raina has gone missing.

Stacy is ready to try and become a full-fledged police officer and leave her job as police services aid. Todd is angry because Stacy didn't tell him of her decision first. The inspectors aren't sure if they are in a gang war when a gang member is murdered. They question a reporter who arrived at the crime scene before police did. Stacy, meanwhile, is shocked when she learns that her parents are coming to see her and that Todd is trying to heal the relationship between her parents and Stacy. Stacy hasn't spoken to her parents since she left home a few years ago. Stacy's parents think she will marry Todd and are shocked at the announcement she makes at dinner.

The Division is on the hunt for a serial killer who left a body in a local landfill. Another missing woman supposedly left a message on her computer that read ""I am okay"". Stacy gets a surprise visit from her former fiance and tries to explain why she left him at the altar. The missing woman's therapist tells Kate that she believes one of her patients may have had something to do with the disappearance of her. Raina meets Johnny's former girlfriend who asks that Raina stop dating Johnny so she can go back to dating him.

Nate and Raina look into the Suicide of Tom Burke who worked at the same facility that Irene (Nate's sister lives. Irene believes Tom was murdered. While Nate questions Bob about Tom Burke and weather Tom took his medicine they find out Bob did not't give out medication. Raina & Nate wonder how Tom got his medicine.

Diane Evans faces her father in court for abusing her as a child. she is shocked when the Judge in the case releases her father because of a Califoronia law that was enacted in 1993. The Judge releases Cale Gillman because the crime was committed before the law was enforced. Diane Evans is angry that the case against her father is dropped now the captain gets word that Diane Evans 's father Cale Gillman was murdered Jinny and Mageda begin to suspect Diane may have killed her father because he didn't go to jail for his crime.

The daughter of a wealthy family is investigated by police who want to understand how members of the Hayes died in the car bomb and who exactly had motive to do it. Florence Hayes filed papers about a month ago that listed three of Florence's a daughter's joined into split the will worth 80 million dollars. Magda also discovered that Jack was taken out of the will because he may have been involved with the plotting to set off the pipe bomb.

The detectives look into a district attorney's past to learn why he was brutally murdered. They soon learn he had HIV and may have exposed virtually hundreds of people. Kate is concerned that Katie Ann isn't walking soon enough. Stacy has to serve 39 subpoenas and encounters a dangerous situation when delivering one. Jinny returns to work after becoming a new mom and learns that Kate isn't immune to being a nervous grandmother just as Jenny feels nervous about being a first time mother and having to let everybody touch her baby.