At the Delancey home, Mrs. Delancey says her husband didn't tell her about any plans for the evening. She and the boys spent the night at their other house. She can't think of anyone specific that might want to hurt her husband, although his success probably caused some enemies. She's worried about the effect this will have on the boys, because their mother died of cancer five years ago, two years before she married their father. Their nanny Ann Barker comes in, and I'm instantly convinced she did it. It's probably not fair of me. She goes to check on the boys (we're told that Zack is particularly distraught), and Holmes takes Watson outside.
Holmes asks if Watson remembers Abigail Spencer, a fifteen-year-old girl who was accused of poisoning her father with nitroglycerine in the early 1990s. She was acquitted, although many people continued to suspect her. And then she vanished. Holmes is convinced it is Ann Barker.
In the interrogation room, Gregson establishes that Abigail Spencer is Ann Barker. She says her father beat her, so she wasn't sad, and that made everyone assume that she killed him. She also denies killing Titus Delancey, which makes Gregson accuse her of being defensive, which makes her explain that it's natural to be defensive when you're being interrogated by the police. I'm surprised her previous experience with being accused of murder didn't teach her to get a lawyer in these situations. She admits that the nitroglycerine is quite a coincidence, but she insists that she didn't do it. She spent last night alone, because she spends most of her time alone. She got plastic surgery to make it harder for people to recognize her, but Holmes spotted her voice. She takes a breath and says that they clearly don't have proof, so she wants to leave.
Bell thinks she's a great suspect, and Gregson says they don't have enough evidence to hold her. Holmes no longer thinks she did it. He now thinks someone is trying to frame her, although Watson says no one is supposed to have recognized her in fifteen years.
Holmes has set up a crazy-person collage wall back at the brownstone. Watson thinks he didn't just recognize Abigail Spencer's voice. He saw the tattoo on her wrist, but she didn't have that tattoo when she was arrested. Holmes says that he was fascinated with her as a boy, and he wrote letters to her full of direct questions. They corresponded throughout the trial, and she mentioned that she wanted to get a tattoo of a phoenix on her wrist. And she didn't recognize the name "Sherlock Holmes" because he was using the name "Shaun." I can understand how a kid might not want to use the name "Sherlock." Anyway, their correspondence gave Holmes insight into her mind, and he believes she did it. She convinced him that her father was abusive and she was responsible for his death. He calls her "an excellent research subject," which is pretty creepy. He didn't tell anyone about the correspondence because the trial was over and he didn't consider her a threat to kill anyone else. He now believes that whoever uncovered Abigail is the killer.