Darby Stone is the elder of two children of widowed Julia Stone, with her only sibling being an infant sister, Beth. Some time before the episode's events, Julia heard Beth crying late one night, and after two failed attempts to get Beth to sleep, Darby went upstairs under the guise of attempting to do the same. Darby turned heel by stabbing her baby sister to death; all the while, Julia noticed that Beth's crying had stopped and went upstairs, only to see that Darby had killed Beth. Darby was shown clutching a knife and was covered in her sister's blood, while also displaying a cold and emotionless expression. Instead of reporting her, Julia destroyed the evidence and had Darby committed to a mental institution with hopes that she would receive the help that she needed, doing so because she feared losing the only remaining family she had.
However, in the episode's events, Darby decided to devise an evil plan to get herself released from the institution, with the first part of her plan beginning with killing fellow patient Renee Caldwell, doing so by beating her to death with a ball-peen hammer in the shower. The murder was committed to lure any police to the institution, and once Megan Hunt and Tommy Sullivan arrived, the evil Darby began her manipulation. She painted herself as an innocent young woman and informed Megan and Tommy that Renee was killed by mistake and she was the actual target. Darby depicted Julia as a twisted villainess; claiming that she witnessed her mother killing Beth and that she had her committed to keep her quiet. She also claimed that Julia was conspiring with Dr. Derrick Malcolm, the head psychiatrist, to have her killed due to Malcolm's inability to control her.
After a visit to Julia revealed that Malcolm was going to subject Darby to electroshock therapy, Megan protests the action. During the procedure, Darby assaulted Malcolm and escaped to Megan's house, where she met Megan's daughter, Lacey; after which, Darby told her twisted version of the events of Beth's murder. As shown via a flashback, Darby claimed that she went upstairs after Beth's crying stopped, and screamed after seeing that Julia had killed Beth, with Julia holding the knife and giving a cold expression as part of her heel persona in the scene. Darby also claimed that she got rid of the bloody clothes, but not the necklace she had on. The necklace was later found and had Beth's blood on it, leading to Julia being taken into custody. Once there, Julia revealed the truth about what happened: Darby actually killed Beth, and she covered for her teenage daughter, because she was all she had left.
Not only was Darby revealed as her sister's killer, but a substance from the institution's heat vent also revealed Darby as Renee's killer as well. Megan was driving Darby to her aunt's house when she received the news from Tommy, and she attempted to keep the truth hidden from Darby. However, Darby figured out that Megan knew the truth, and she jumped out of the moving car and ran towards a policeman with claims that Megan abducted her. Later on, the twisted villainess stabbed the policeman (non-fatally) with a knife, but Megan managed to hold Darby at gunpoint, leading to her arrest. During interrogation, Darby boldly admitted to killing Beth, who she referred to as a whiny brat, and lambasted her mother for committing her. She also boasted about how she manipulated Megan, who informed the sinister sociopath that she would be going away for a long time.
- “You’re a good liar, Dr. Hunt... but not as good as me.” (Darby to Megan just before the former jumped out of the latter’s car)
- “Beth was a whiny brat. Trust me; I did the world a big favor. (Megan: “And you ruined your mother’s life.”) Yeah? Well, she tried to ruin mine by throwing me into that freak zoo. (Megan: “She was trying to get you help.”) Mom was never gonna let me out of there... so I had to do it myself.” (Darby coldly confessing to killing both Beth and Renee, cementing her reveal as a murderous and sociopathic villainess)
- “All I had to do is find someone to believe it. I chose you. (Megan: “Your mother’s right. You’re a sick little girl.”) And your daughter was right. You act all tough, but you really just wanna help people. It was so easy to make you sorry for me. (Megan: “Now I feel nothing for you... and you’re going away for a long time.”) We’ll just have to see about that... won’t we?” (Darby’s last words to Megan prior to being sentenced to an unknown amount of time in a prison offscreen)