15-year-old Renee Caldwell is found dead in the shower at the mental health facility where she had been committed. Megan says the location of the murder allowed for much of the evidence to literally go down the drain. Dr. Derrick Malcolm (guest star Craig Bierko) says the victim was a paranoid schizophrenic. He insists on being present during any interviews Tommy conducts. One patient, a young girl named Darby, slips Megan a note so they can have a private chat. She says she saw a man climb down a heating vent to kill Renee, who wasn't the intended target. Darby believes the killer was actually after her.
The victim was killed with a blunt instrument, perhaps a ball-peen hammer. A substance that looks like petroleum jelly is found on the victim's shoulder. Tommy learns that Darby's little sister was murdered a few years ago. Dr. Malcolm refuses to let Megan talk to his patient. Actually, Darby couldn't talk anyway, as she's been heavily sedated. Megan thinks Malcolm is hiding something. There's also evidence that someone did climb in through the heating vent. A gooey substance like the one found on the victim is also present in the vent. Perhaps Darby was telling the truth. Perhaps she isn't crazy at all.
A lucid Darby tells Megan that her mother hired someone to kill her. She's trying to keep her quiet because she saw her kill her little sister. Darby was committed because she wanted to tell the police. She says Malcolm is in on the cover up. Megan believes this girl is telling the truth. She gets her mom, Julia (guest star Sharon Lawrence), to admit that there was no documentation of mental illness before she had her daughter committed. She also learns that Darby is being treated with electroshock therapy. Megan is helpless to stop this. Darby, on the other hand, isn't. She assaults Malcolm before the procedure and escapes from the institution.
Megan gets extremely uneasy when she learns that Darby is at her apartment alone with Lacey. Both girls are fine when she arrives. Megan promises Darby that she won't let anything happen to her. She admits to Tommy that she wants to protect her. Darby recalls the night her sister died. She caught her mom sitting on the bed with as bloody knife in her hand. She got rid of her bloody clothes, but not the pearl necklace she was wearing. Ethan finds blood inside one of the pearls. It's a match to the dead little girl. Julia says she did destroy evidence that night. She was trying to protect Darby after she killed her sister. Julia was protecting the one child she had left.
Megan volunteers to drive Darby to her aunt's house. Meanwhile, Tommy learns that the gooey substance from the heat vent came from inside the institution. Darby used it the night she killed Renee. Tommy tries to warn Megan. Darby is hip to what's happening. She jumps out of the moving car, flags down a cop and accuses Megan of trying to hurt her. She stabs the cop, but Megan is able to grab his gun. She learns that Darby is a sociopath who targeted Megan because she knew she would help her. Dr. Malcolm apologizes for not realizing how dangerous Darby was. He also lets Megan know that people are messy. Even doctors.
Megan lets her mom know that a partial print was found on her dad's suicide note. She wants an exhumation notice to dig up the body. Joan is not willing to authorize this. Dr. Charlie Stafford (guest star Luke Perry) offers to help. Megan is pleasantly surprised that he trusts her instincts. In other news, Megan is approached by Riley about her relationship with Tommy. She gives the pretty young officer the go-ahead to pursue her old hockey date. Too bad Tommy is only interested in Megan. He's upset that she keeps finding reasons to push him away. Megan says they should put the past to rest. She believes it's for the best.