You told the police you’d killed Mary, told them how and where you did it. But the woman you described was Martha—the woman you knew they’d find alive at 15 Megson Crescent. That was the point where you can’t have been sure. It was a gamble: either she’d tell them everything, produce whatever proof she had, or she wouldn’t. She’d say nothing. And the police would dismiss your story as the ravings of a deranged man, a man who could look them in the eye and insist that he’s murdered somebody who isn’t dead. You wanted them to think you were crazy. You didn’t want to go to prison.

You regretted telling me you’d killed Mary Trelease as soon as the words were out and you saw the horror on my face. But you couldn’t take it back, not something as big as that. You couldn’t say you were joking. I wouldn’t have believed you. I could see the state you were in. Your only hope was to turn your confession into one you knew could be disproved—disproved by the existence of a woman calling herself Mary Trelease.

As much as you wanted to protect yourself, you also wanted to confess. And you did: finally, you went to the police and told the truth. Even when you had to lie, when you had to withhold so much of the story that you ended up telling a different story altogether, you were still able to say the main thing that was true: that you’d killed Mary Trelease, that you’d strangled her in bed, in that room. It must have felt good to say it, after so many years of guilt and silence. Unburdening yourself, but with a safety net in place to give the lie to your confession—the presence of a real live Mary in the house where you told the police they’d find her body.

She didn’t tell them what she knew. That would have meant handing control over to them, and there was no way she’d do that. You saw she hadn’t done it—no detectives came back to you to ask about the other Mary Trelease, the real one. But it still wasn’t over. Martha Wyers wasn’t going to disappear; you knew how doggedly clingy she was, how determined she was to latch on to your life as if it was rightfully hers. She was still there, at 15 Megson Crescent. She still knew what you’d done. It would never have been over, not unless you’d killed her, and you couldn’t do that. You weren’t a killer. I don’t know why you killed a woman years ago, but I know you’re not a killer.