Everything's gone strange. Merlin claims magic involved. Most recent murder is "declaration of war". Please investigate.

Sherlock didn't send the text. He had addressed it to Mycroft, but he could already imagine his brother's reaction. Mycroft Holmes would be as incredulous as he had been. Indeed, he might think that his little brother had finally cracked. Anyway, he didn't yet know who Merlin was – well, not formally. No doubt he already knew all about him. But they hadn't met.

And, furthermore, Mycroft knew very little about this case.

He had addressed the text to himself, really. Sitting at the feet of the statue of Abraham Lincoln, he surveyed the lawn and the Palace of Westminster from a distance, without really seeing them. The area had been cordoned off, so there wasn't the usual constant stream of traffic crawling round the corner towards the Thames; it seemed curiously silent.

Merlin was still talking with Lestrade. The detective inspector didn't look terribly convinced by whatever it was he was saying. The mutilated corpse was hidden beneath a blanket. The witnesses were still sobbing in the van. It looked like any other crime scene, but Sherlock knew that it wasn't. Normally that thought would have exhilarated him. Today it did the opposite.

Sherlock Holmes didn't like feeling defeated. He especially didn't like feeling that a case was beyond him: that was what annoyed him so much about this one. That gangly English Lit student knew more about it than he did, and that was just salt in the wound. He could however sense, with whatever subconscious sense it is that deals with the supernatural, that something wasn't at all right with the whole business, and that Merlin's statement about a declaration of war might not be so far-fetched as everyone seemed to think.

He glanced up at Abraham Lincoln, who just smiled enigmatically at him, and went over to where a police officer had left his bicycle. He needed to pay a visit to a certain club on Pall Mall.

The crash of a bicycle being thrown against a bollard perhaps made a couple of the gentlemen look up, but none of them really reacted. They did not even deign to glance towards the scruffy young man whose presence always seemed to precede some great disturbance, and let him go straight to the Strangers' Room (though of course he was far from a stranger to the Diogenes Club), where he would meet his brother, the Acting King of England, or whatever it was he called himself these days.

The gentlemen of the Diogenes Club didn't care for Sherlock and Mycroft Holmes. They much preferred good pipes, fine tobacco and a copy of their newspaper of choice, freshly ironed. Such is the life of the rich and peculiar.

Mycroft Holmes was of course a member (and possibly founder, nobody could quite recall) of the Club. But he smoked cigarettes, and didn't much care for newspapers, and so on this particular morning, when his briefcase looked decidedly empty, he welcomed any excuse to leave the lounge, even if it was to talk to his brother.

Sherlock Holmes had arrived fresh from a case. Probably that disturbance over by the Houses of Parliament. He had come by bicycle, which surprised Mycroft – it was probably the quickest way of reaching Pall Mall from Parliament, but Sherlock didn't much like riding, and would have forgotten long ago how to if it wasn't one of those unforgettable skills. The shambolic nature of his arrival suggested that he was in a hurry. But now, sitting in a plush chair in the Strangers' Room, his eyes travelling carelessly over the panelled wall, Sherlock Holmes did not seem in much of a hurry to do anything.

'We are in the Strangers' Room; talking is at least permitted, if not necessarily preferable,' Mycroft reminded him after a minute.

Sherlock furrowed his brow, and, after thinking for a moment, slid a handful of photographs across the table.

Mycroft picked up the first, which showed the burn mark on the first corpse. He grimaced almost imperceptibly. 'I was aware of the details of the case.'

He picked up the second, which was of the second corpse, and which incorporated the mysterious message etched on the wall. 'I was aware of this as well.'

Then his hand fell on another of the photographs, which was an instant Polaroid one straight from the most recent crime. It showed the grisly detail of the mutilated face, and the way the shadows fell could only emphasise the excess skin that seemed to have grown in an instant on his visage. Mycroft's own face contorted. It was evident that he had seen nothing like it before.

'This is the case at Parliament Square?'

Sherlock nodded.

'And the details of the case –?'

'The witnesses say that the victim was struck by some sort of fit. He collapsed face first. When one of the witnesses turned him over, he looked like that. He had died almost instantly. She confirms that he did not look like that before.'

This final statement was the most important. Mycroft sat up straight, his jaw set.

'My new flatmate, Merlin Ealdor, is currently at the scene. He came with me earlier. His reaction to the murder was – surprising,' Sherlock continued, and summarised in a few words what had happened and what Merlin had said. When Mycroft's eyes questioned him, he added a few comments about the other crimes. Mycroft had evidently realised the connexion with the name on the wall, because he fixed his eye on that photograph for a long while.

'We are left with two hypotheses,' Sherlock said. 'Both of them would suggest that Merlin is involved with the series of crimes, either on "their" side, or against "them". One hypothesis states that this is a case about which we don't know enough, and which has a perfectly rational foundation. The other states that – well, Merlin is telling the truth.'

Mycroft did not quite look sceptical at this remark, but his eyebrow twitched slightly. It occurred to Sherlock that he hadn't told Mycroft about Merlin performing magic in front of him. He wasn't sure he ought to. He still couldn't quite believe it himself.

But, after a moment, dismissing whatever doubtful thoughts had crept into his mind then, he said simply: 'A perceived declaration of war, in whatever circumstances it was made, ought to be taken seriously. You will accompany me to Parliament Square, I presume?'

And with that he got up and left the room. After a moment's hesitation, Sherlock pocketed the photographs and followed him.