Sherlock never told John that he worked It out because John would have insisted on joining him. But he knew, and he was there, on New Year's Eve, at the Palace of Westminster.

Even though it was a bit rainy, people would always come to the city on New Year's Eve to see the big Christmas tree in Trafalgar Square, to have a drink and a party, and to hear the Great Bell, commonly known as Big Ben, strike twelve for the last time of the year. This year it wouldn't.

From the Palace, I could look out over the Thames. I'd expected him to get off the tube at Westminster, but he hadn't. My people pointed him out as he came walking across Westminster bridge, just as Big Ben started to strike. People with drinks in their hands all over the city would count with it. One, two, three, four, five... Now I finally saw him between the spectators, in his black coat and blue scarf, walking across the beautifully illuminated bridge. Six, seven, eight, nine... He was at the end now. Like everyone else, he was looking up at the clock tower. Ten, eleven, twelve... a massive bang from the firework display greeted the new year, but not enough to drown out the last strike. Thirteen.

I went up into the clock tower (officially she's called the Elizabeth Tower but I refuse to call her that). It is over three hundred steps. My people knew why we were here and they would let him in. It took him a while to get there, I was already at the clock when he finally entered the tower.

'Mycroft!' he screamed, looking upwards onto the long spiral staircase. I didn't answer. He knew I was there. In the small space between the western clock and the panel that illuminates the transparent white glass that forms the back of the clock, one could hardly call me hidden. My silhouette was even visible from the other side of the Thames.

I heard his footsteps as he walked up the stairs. 'I know everything!' he yelled up into the tower. 'The students got in your way, didn't they?' A pause as he walked further, his footsteps slowly became louder. 'They were making a case against you, weren't they? They'd end your career!' I could hear him panting now, maybe from climbing and yelling at the same time, or maybe because he was biting back tears. 'So you just killed them? Just like that?' I could hear him stop and catch his breath. He was almost at the clock. I resumed my position at the far end of the western clock looking at the white glass. As he finally reached my level I turned to him.

'Dear brother,' I said, 'It's way more complicated than that'.

He looked at me, his face a mixture of anger, pain and disbelief. 'You better explain this to me.'

'I will, Sherlock, I will.' With a gesture of my hand, I invited him to step towards me and to look at the clock. Apart from a maintenance hole, the white glass was semi-opaque; you could see the hands and the dials through it. I knew that from outside, both our silhouettes would be visible now. I snapped my fingers and on that sign, with a bang, the lights that illuminated the clock from behind us went out. We stood there in complete darkness until our eyes slowly adjusted. Through the glass, the lights of the city and the fireworks became visible.

'Look, Sherlock,' I said. 'What do you see?'

'London.'

'There it is, stretching out in front of us with all the lights and fireworks and even the big silly Christmas tree.'

Sherlock had found the maintenance hole and looked through it. 'You didn't just bring me here to show me London.'

'This is the free world you're looking at. A free country. The population of London is over eight million. You can see some of those people now. You can see some of their houses and the places where they work. Eight million free people. And sometimes, their freedom needs protection.'

'Explain to me the link between protecting freedom and killing five students.'

I sighed. 'Sherlock, what do you remember of our brother Sherrinford?'

'I was eleven when he died, Mycroft. For me he was just a big guy with blue eyes.' Sherlock paused. I saw him frown as he tried to access the old memories. 'He smoked, he did experiments, he was a bit unpredictable, I remember.'

I shook my head. 'Sherlock, our brother was an absolute psychopath. A narcissist who lived for power.'

'Bit like you then.'

'A bit not like me. Have you ever bothered to find out who those students socialised with?'

'I was more interested in the forensics.'

I rolled my eyes in the dark. 'I feared as much. That is why I brought you a picture.' From my coat pocket, I produced the picture and a small torch and gave them to him. Sherlock looked; the picture showed six young men at a bar, laughing with their drinks.

'Do you recognise the sixth?'

Even if he didn't, our conversation had already primed him to the right answer. In the middle sat a man with short dark hair and bright blue eyes.

'Sherrinford.'

'Correct.'

'He studied maths. He was the best of his year. Were those his friends?'

'If such a person would be capable of having friends, perhaps. He was a genius and a real star. He had already been offered jobs here at all the central intelligence agencies. I think they believed they were his friends. That is until he learned what they came to know about him.'

'What did he do?'

'This was 1987. The internet wasn't yet invented but computers had been around for a while. All signs suggested that at some point, data processing and sharing would become much easier and much more widespread than ever before. Our brother knew that and he hatched a plan so strange that no one would even think of it. He was slowly moving himself into a position where all that information would need to be filtered by him.'

'He was a student.'

'He was what they called at that time a whizz kid. He already worked for MI5 in secret and he had been paying lobbyists for five years. No one could catch him, except another genius.'

'Why did he do that?'

'To quote our friend Orwell again: Power is not a means; it's an end.'

I looked at my watch, it was almost quarter past twelve. I reached into my breast pocket and took out two sets of earplugs.

'The quarter bells will play in a moment. You'll need this.' I said dryly.

He was smart enough not to protest. We put the earplugs in and waited until the bells stopped playing before taking them out again.

'You know what those chimes sing every quarter of an hour?'

He shook his head.

'All through this hour,

Lord be my guide,

And by Thy power,

No foot shall slide.'

I followed his eyes downward. 'How's your footing Sherlock?'

He looked at me. There was no fear in his eyes. Just anger.

'Why did the students have to die?'

'Oh, Sherlock, you were always so slow. They were not my doing. You remember that Sherrinford was always working with almond seeds?'

'Almond seeds.' Sherlock looked at me in disbelief. 'So often used to make cyanide. How could I not see that?'

'Because you were too young at that time. Later on, you never made the connection.'

Sherlock didn't seem to register it.

'It was him. The message was for you.'

'We fought. We fought all the time but at some point I realised that he wasn't going to change his mind. Then I talked to his friends. They were all really smart people and they had connections. But one thing I didn't foresee: human sentiment. Instead of isolating Sherrinford, they tried to talk to him. He immediately understood that I was behind it. That sealed their fate.'

'They died in a really obvious way, yet no traces of murder. It was a threat.'

'Killing five people you know, just to make a point. At that moment, I truly understood his real nature. There was only one option left to me.'

'His cigarettes. With his own poison. You murdered your own brother.'

'It was either that or our whole country in great jeopardy.'

He looked at me, for a moment too stunned to say anything at all. Then he spoke with a soft voice I rarely ever hear from him. 'So that's the utilitarian solution.' He breathed in slowly. 'The needs of the many outweigh the needs of the few.'

I sighed. 'That's the only sensible position. Having a great intellect creates great responsibility. I gestured to the city. 'I've dedicated my life in service of these people. I am a guardian.'

'You are a murderer.' His voice was ice cold. We just stood there, behind the clock, looking at the lights of the city. Finally, he got to his feet. 'This is the last you will ever see of me,' he said in the same cold tone. He walked away, the sound of his footsteps on the long staircase slowly growing fainter.

I waited until he was gone before I followed. As I slowly walked down, I wondered how things would change now. We don't see each other often but somehow he had always been a fixed presence in my life. Usually, he adds a little spice to it, not always pleasurable. My life without him would be more predictable and most certainly more efficient. Visits to parents would need to be planned carefully now. Though I could always give up on Christmas dinners. The thought didn't cheer me up. Sherlock and I never fought because we hated each other. I didn't sleep much that night and the following morning I got a compulsion I never have. I desperately wanted to visit the grave in the silence of the first morning of the new year.

I don't see his reflection in the stone, I just hear his footsteps. Of course I would recognise his footsteps although in all honesty, for a moment I think my brain is playing tricks on me. I hide my surprise and continue to stare straight ahead, at the gravestone, until he is finally caught in the reflection. I don't say anything as he stands next to me, vaguely ghost-like in that ridiculous long coat of his.

'I don't even remember his face.' he says.

'You were young,' I say without looking at him. Sherlock and I never needed many words.

'I was eighteen, Sherrinford was twenty three, you were only eleven.'

'So you do remember his face.'

'I see it every night.'

Sherlock doesn't respond to that, doesn't even look at me but focuses on the gravestone itself. Sherrinford Holmes. I never told Sherlock that he was here.

He finally breaks the silence.

'So how did he do it?' Curious as ever.

'Mince pies. He gave them all one for the way home. They got peckish between four and five. One was still driving.'

'I see.'

'We're not like him, you and me.'

'I know. That's why I'm here.' He chuckles. 'You're shit at being a utilitarian.'

I smile with him. 'You know philosophical ethics is only a clumsy tool to rationalise what we feel inside.'

We don't say anything for a long time. When he finally speaks, his voice is raw.

'Some people, when they think of their families, they think of dinners and making jokes. When I think of my family, I think of unspoken pain. I never understood.' He looks at me fiercely now. 'I never understood, Mycroft.'

I do something I never do. I grab his hand.