Mycroft's office exuded organisation. Everything had a place, and was tucked into its place with the utmost precision. His two armchairs looked as if they had been freshly groomed. The fireplace was immaculate, the desk shining; even the rows of folders had been sorted into rainbow, and then alphabetical, order. If Sherlock had spent a couple of days tidying 221B, it would have almost looked like this.

It was certainly a strange place to be discussing the imminent destruction of England.

Merlin and Mycroft were on their second cup of tea, and Mycroft, though still a little doubtful about the whole matter, had agreed that precautions needed to be taken. He then moved on to the second point for discussion – the two cases.

Mycroft had somehow got hold of all of the details from both. He had read them numerous times over, and they were now imprinted on his mind. Therefore he hesitated a moment, and then said:

'Magic exists.'

'Yes,' replied Merlin, after a moment.

This was the one cog that seemed to be sticking a bit. Mycroft half-smiled, and sipped from his cup of tea. 'No doubt you could prove it. It is merely a – surprise.'

'Admittedly,' said Merlin, 'I thought it was as good as extinct. I haven't met another magician since 1956. And he wasn't a very good one.'

Mycroft furrowed his brow a little.

'Though magic has changed. Magic evolves. It isn't done in the same way as it was when I learnt it. No – when I perfected it. I never had to learn magic.' His eyes drifted a little round the room as, for a moment, a memory overtook him. 'Though I believe,' and he faltered, 'that I am still the most powerful magician in existence. I always have been. I don't mean to boast,' he added hurriedly. 'It's a curse more than anything else.'

Mycroft seemed to be only half-listening to him – not out of boredom, but merely confusion, and also a feeling that he was rambling a little too much – so Merlin fell silent again.

He looked towards the clock on the mantel. It was nearing twelve o'clock. He'd been here more than an hour. Not particularly long, perhaps, but when the world could end at any moment, one does become a little conscious of the passage of time. He was about to excuse himself – see where Sherlock had got to – when there was a ping from his pocket, and he extracted a brick of a phone.

'Sorry,' he said automatically to Mycroft, opening the text message that had appeared on the screen.

Merlin. There's a man in the flat claiming he knows you. Says he's called Arthur Pendragon. SH

The phone nearly slipped from his fingers, but Merlin managed to keep his composure just long enough to excuse himself and leave the room. Then he took off at a run.

It was one thing to be in the same room as an ancient magician. It was another to be in the same room as a legendary king.

Sherlock's knowledge of the story of King Arthur was sadly lacking, but his conversation with the man himself had rather filled him in on a lot of the details. He had to admit to being mildly fascinated, though a lot of it was a bit irrelevant given the current circumstances. It turned out that Arthur hadn't known where to find help, but had been directed towards Baker Street by someone who always tended to send dishevelled-looking fellows in that direction. (Which explained a lot, to be honest.)

At length a discussion of Merlin had come up; Sherlock's description of the boy had matched Arthur's exactly, and so he had sent for Merlin, not quite realising, perhaps, what a reaction this would elicit.

They had just then, after an hour of discussion, got onto the details of the case: Sherlock had meant to abandon it, but it seemed that he could no longer think of doing so. He explained everything to Arthur, who looked bewildered, but managed to keep up; then, whilst detailing the third (and most peculiar) murder, Sherlock handed over the photograph of the body, which did a better job of explaining than he could ever have done.

'I've seen this before,' Arthur said at once.

That was precisely what Merlin had said, on seeing the mutilated corpse. Sherlock shuddered a little.

'A – a long time ago: what year did you say it was?' Arthur asked. Sherlock told him. 'The witch Morgana –'

'Merlin's told me this bit,' Sherlock said, trying to disguise the fact that his heart had leapt. One person telling him such ridiculous theories was dismissible, improbable. But another confirming the same story –

'Where is Merlin, anyway?' asked Arthur after a moment.

'He was at Mycroft's,' Sherlock said, 'but I texted him –'

Scarcely had he finished his sentence when they both heard a taxi pull up outside the door. Footsteps on the pavement pattered into the house, and someone came running up the stairs. Merlin did not hesitate to fling open the door to 221B and enter; his hurried pace was halted immediately at the sight of the figure in the second-best armchair.

This time he did drop his phone. Then, still staring at Arthur, he clung to Sherlock's chair and dropped to his knees. He covered his face with his hands but they both heard him break into sobs.