Lord Darcy’s trip from the Palace du Marquis to the Tower of London was uneventful. The cab clattered out of Mark Lane, swerved, and descended Tower Hill. In Water Lane, at the gate, it stopped. Lord Darcy stepped out.

A heavy, whitish fog drifted through the bars of the great iron fence and clung to the shadows of the Gothic archways. There was a fading sound of bells as the ships on the Thames moved through the mist-laden waters. The air was muggy, and a faint smell of marine decay drifted over the wall that formed one side of the fortress. Lord Darcy wrinkled his nostrils at the aroma that assailed them, and then walked over the stone bridge that led from the Middle Tower to another tower — larger and gray-black, with a few whitish stones here and there in its walls. There was another archway, then a short, straight path, and then Lord Darcy turned toward the right and entered St. Thomas’ Tower.

Within a few minutes, the Warder was unlocking the door to Master Sean’s cell. “Call me when you wants to leave, your lordship,” he said. He left, closing the door and relocking it.

“Well, Master Sean,” said Lord Darcy with a spark of humor in his gray eyes, “I trust you are enjoying this idyllic relaxation from your onerous duties, eh?”

“Hm-m-m — yes and no, my lord,” said the tubby little sorcerer. He waved a hand at the small plain table on which his carpetbag sat. “I can’t say I enjoy being locked up, but it has given me an opportunity to experiment and meditate.”

“Indeed? Upon what?”

“Upon getting in and out of locked rooms, my lord.”

“And what have you learned, my good Sean?” Lord Darcy asked.

“I’ve learned that the security system here is quite good, but not quite good enough. To hold me in, I mean. The spell on that lock took me ten minutes to solve.” He picked up a small wand of gleaming brass and twirled it between thumb and forefinger. “I relocked it, of course, my lord. No need to disturb the Warder, who’s a decent sort of fellow.”

“I see you regained possession of your bag of equipment easily enough. Well, one could hardly expect an ordinary prison magician to compete with a Master Sorcerer of your capabilities. Now pray be seated and explain to me in detail how you came to be incarcerated in one of London’s oldest landmarks. Omit no detail.”

Lord Darcy did not interrupt while Master Sean told his story. He had worked with the little sorcerer for years; he knew that Sean’s memory was accurate and complete.

“And then,” Master Sean finished, “Lord Bontriomphe brought me here — with, I must say, sincere apologies. I can’t for the life of me see why the Marquis should order me locked up, though. Surely a man of his abilities should be able to see that I had nothing to do with Sir James’ death.”

Lord Darcy scooped tobacco from a leathern pouch and thumbed it into the gold-worked porcelain bowl of his favorite pipe. “Of course he knows you’re innocent, my dear Sean,” he said crisply. “My Lord Marquis is a parsimonious man and a lazy one. Bontriomphe is an excellent investigator, but he lacks the deductive faculty in its highest form. My Lord Marquis, on the other hand, is capable of brilliant reasoning, but he is both physically and mentally indolent. He leaves his own home but rarely, and never for the purpose of criminal investigation. When he is pressured into doing so, My Lord Marquis is perfectly capable of solving some of the most intricate and complex puzzles with nothing more to work with than the verbal reports given him by Lord Bontriomphe. His mind is — brilliant.” Lord Darcy lit his pipe and surrounded himself with a cloud of fragrant smoke.

“Coming from you,” said Master Sean, “that’s quite a compliment”

“Not at all. It is merely a statement of fact. Perhaps it runs in the blood; we are cousins, you know.”

Master Sean nodded. “At least the laziness doesn’t run in the blood, my lord. But why lock me up because he’s lazy?”

“Lazy and parsimonious, my good Sean,” Lord Darcy corrected the sorcerer. “Both factors apply. He has already recognized that this case is far too complex for the relatively feeble powers of Lord Bontriomphe to cope with.” Lord Darcy smiled and took the pipe from his lips. “You said a moment ago that I had complimented my lord’s brilliancy. If that is so, then he has, in his own way, paid the same compliment to me. He is mentally lazy; therefore, he wishes to get someone else to do the work — someone competent to solve the problem with the same facility with which he would do it himself, were he to apply his mind. He has chosen me, and I flatter myself that he would not have chosen any other man.”

“That still doesn’t explain why he locked me up,” Master Sean said. “He could have just asked you for assistance.”

Lord Darcy sighed. “You have forgotten his parsimony again, my good Sean. Were he to ask His Royal Highness of Normandy to spare my services for a short while, he would be obligated to pay my salary from his own Privy Purse. But by incarcerating you, he deprives me of my most valued assistant. He knows I would not suffer you to be imprisoned one second longer than necessary. He knows that putting you in the Tower would force me to take a leave of absence, to solve the case on my own time, thereby saving himself a pretty penny.”

“Blackmail,” said Master Sean.

“ ‘Blackmail’ is perhaps too strong a word,” Lord Darcy said thoughtfully, “but I will admit that no other is quite strong enough. That problem, however, will be taken care of in its own time. At the moment, we are concerned with the death of Sir James.

“Now — what about the lock on Sir James’ room?”

Master Sean settled himself deeper into his chair. “Well, my lord, as you know, most commercial spells are pretty simple, especially those where more than one key has to be used, as they have in a hotel.”

Lord Darcy nodded patiently. Master Sean O Lochlainn had a rather pedagogical habit of framing his explanations as though they were lectures to be used in the training of apprentice sorcerers — which was not surprising, since the tubby little master magician had at one time taught in one of the Sorcerers’ Guild’s schools and had written two textbooks and several monographs upon the subject. Lord Darcy had long ago formed the habit of listening, even though he had heard parts of each lecture before, for there was always something to be learned, something new to be stored away in the memory for future reference. Lord Darcy did not have the inborn Talent necessary to make use of the Laws of Magic directly, but one never knows when some esoteric bit of data might become pertinent and useful to a criminal investigator.

“The average commercial spell uses the Law of Contagion, so that every key which touches the lock during the casting of the spell will unlock and lock it,” Master Sean continued. “But that means a relative weakening of the spell. An ordinary duplicate key won’t work the lock, but any good apprentice o’ the Guild could break the spell if he had such a duplicate. And any Master could break it without the key in a minute or two.

“But a personal spell by a Master uses the Law of Relevance to bind the whole lock-and-key mechanism together as a unit — one key, one lock. The spell is cast with the key in the lock, so that the binding considers the key simply as a detachable part of the mechanism, if you follow me, my lord. No other key will work, either to lock or to unlock the mechanism, even if it is so physically like the proper key that they couldn’t be told apart.”

“And Master Sir James’ key-and-lock had that sort of spell on it, eh?” Lord Darcy asked.

“That it did, my lord.”

“Could a Master Sorcerer have removed the spell?”

Master Sean nodded. “Aye, that he could — in half an hour. But look what that would entail, my lord.