Once I finished my chemistry degree, I moved into a tiny flat in a grimy corner of London. I had no desire to join the official police force and be bound by its endless regulations or to become a teacher of any kind, or, I really don't need to add, to join the civil service, where my brother was already rising with astonishing rapidity. By then, my parents had, I believe, given up the notion that I was likely to become anything conventional.

No, those were the days when I set out to learn the ways of the city that is my temperamental mistress to this day. One week I would load lorries with huge boxes filled with machine parts, another I might be typing correspondence for a law firm, the next delivering take-away food. My longest stint in one job was as nanny to seven-year-old twins, a boy and a girl. I was very good at it, as I always am with children. They haven't learned to be stupid like their parents yet. I liked that position; it gave me ample opportunities to observe the wealthy of London and to understand their world.

I walked the alleys and streets; I was mugged; once a woman saw me and tried to recruit me to become a model. Said I had an interesting face. It was one of the most absurd conversations of my life, but I loved London, and she loved me. We were strangely suited to each other, like the yin to the yang. I was the introvert, she the extravert. I was solitary, she dizzyingly sociable. Most of all, she contained the crimes that fueled my passion for detection. I am not such a heartless man as to wish for evil that doesn't already exist, but if people are going to commit crimes, I'd rather they do it near enough to me that I can find them out.

Gradually, people began to notice me. I discovered embezzlement at a company for which I spent a month doing data entry. I helped a divorce attorney find evidence of an affair. I told a shipping company why its shipments were always just short. My real emergence, however, came when Andrew Porter went missing.

As I mentioned above, I was a nanny for several months, to two children of very wealthy and extremely busy parents. My duties included readying the children for school and taking them, picking them up, and then staying with them for most of the evening and assisting with their schoolwork. Sometimes I was required past midnight, but during the day, I had ample time to explore as I wished. On school holidays, I took Andrew and Alex to the library and the science museum and the aquarium. They asked questions, and I knew the answers. We got on very well.

I will allow a tangent here, knowing the sort of thing you'd ask. I did not have trouble with the children. I don't believe much in intuition, but when it comes to humans under the age of fifteen, I possess a kind of sixth sense. The two As were fond of me, but they did not try to cross me; they knew that I was immovable. Besides, I didn't ask them to behave in illogical ways. As I constantly tried to tell their mother, children respond to logical routines. She only ever laughed and said she was glad to have me, which was infuriating. The As still write to me through my website. Andrew is and artist, and Alex is a police detective. She claims it's because of me.

I had been with the Porters for six months when Andrew went missing. I left him in his parents' hands at 9:00 one evening, but when I arrived at 7:00 the ext morning to feed breakfast to the twins, I found the mansion in chaos. Andrew and Alex did not share a room, and his absence had not been discovered until his sister had gone into his room that morning to wake him up.

Mrs. Porter was crying hysterically, her husband was trying to talk to a police inspector, and I saw that a team had already been into the boy's room and was scanning the house for evidence. More like destroying evidence, really. Their methods weren't any better than they are now.

No one at all was paying any attention to Alex, who was small and pale, with red hair and freckles that went across her nose. I found her in the linen closet, hiding behind the clean was a large closet, so I sat down beside her, and she climbed into my lap and put her arms around my neck.

"The policeman asked me what happened," she whispered. "I don't like him."

"I don't like him either," I agreed, " He's quite stupid, but what did happen? Can you tell me?"

"Me and Andy always watch cartoons before you come, but I watched a whole one by myself, and he never came down. I went up to see if he was still sleeping, and he wasn't there."

"All right," I answered. "I need you to do something very hard. Do you think you can do it for me?"

"Ok," she said, her eyes wide.

"Close your eyes," I said. She obeyed immediately, and I cradled her head against my chest and spoke softly. "Now, Alex, think about last night. Imagine that you're lying in bed. Keep your eyes shut, and tell me about it."

"Mummy and Daddy put me to bed, and I don't remember anything else. Are you mad?"

"Shh," I said, "keep thinking. Did you hear anything? Smell anything?"

She was quiet for a moment. "Candy."

"Candy?" I asked, keeping my voice calm so I didn't upset her.

"Candy, like the fluffy kind," she said. "I smelled it."

"You're a clever girl," I said, getting up, "but we have to hurry."

I swung her up into my arms and walked as quickly as I could without running. "I'll take Alex for the day," I said to her mother on the way out, meeting with no objection from the frazzled woman.

I took the little girl to the Tube, my brain working as fast as I could possibly make it go. "Sherlock?" she asked, when we were seated side-by-side in a traincar, "is whoever took Andy going to take me?" She was clever enough to realize that kidnapping was the obvious implication.

"No," I said. "I'm going to find Andrew, and both of you are going to be safe." She looked terrified, so I pulled her close and wrapped the edge of my coat around her.

"All right," she said, and I knew she believed me.

I took her to the carnival, of course, the dodgy one her mother had taken the children to against my advice. From there, it was a depressingly pedestrian crime. Andrew had been kidnapped by the idiot who sold cotton candy, who was hoping to extract ransom money from the parents. The boy was rescued, unharmed except for having eaten about five times as much cotton candy as anyone should consume in a lifetime, and the perpetrator went to prison.

The Porters were thrilled, and they insisted on telling the story to all of their friends. The newspapers and blogs picked up the story—a "feel-good," they called it. The police noticed me, too—particularly the inspector on the case. Perhaps you've surmised that his name was Lestrade.

Ironically, I hadn't done much deduction on the case, other than paying attention to a child and figuring out what she meant, but soon after that, I received enough inquiries from prospective clients that I was able to pursue detection as my sole profession.

That was when my Mind Palace became more than a personal exercise; it became my livelihood. No longer did I restrict my concepts to a single building, even one as big as a university. The city herself became my palace. Her streets, her pubs, her alleys and corners, all lived in my memory, and into them I poured the things I needed to remember, like water filling a pitcher to overflowing.

A month after I'd entered private practice, a case called for me to visit St. Bart's. It's a glorious building, as you know. I walked the halls, trying to extract information from its assistant director, and I finally contrived to be taken to the morgue. I had no premonition whatsoever of the part it would play in my future.

I thought, as I stepped over the threshold, that my Science Girl would like to live in it. But of course, she was already there.