Monk obeyed. Very briefly he described Mary Havilland's fall from the bridge with Toby Argyll, then his discovery of James Havilland's earlier death and the course of the investigation that had led to the arrest of Aston Sixsmith.

"Surely you don't want me to defend Sixsmith," Rathbone said incredulously.

"No… at least not to act as defense for him," Monk replied. He was beginning to wonder if what he was intending to ask was impossible. Again, fury at Argyll washed over him, and a sense of helplessness in the face of the skill with which Argyll had manipulated both Sixsmith and the police into the position he wanted them in. Monk could picture Argyll's angry, slightly arrogant face marred by grief as if he had seen him only moments ago. "I want you to prosecute Sixsmith, but in such a way that we get the man behind him," he answered Rathbone. "I don't think Sixsmith had any idea what the money was for. Argyll told him what to do and he did it, either blindly or out of loyalty to the Argylls, believing it was for some legitimate purpose."

Rathbone's fair eyebrows rose. "Such as what, for example?"

"Tunneling is a hard trade. I don't say he wouldn't cut corners or pay bribes to some of the more violent of those who know the sewers and the underground rivers and wells. I don't know."

Rathbone thought for a moment or two. Clearly his interest was caught. He looked at Monk. "You believe the elder Argyll brother used Sixsmith to pay an assassin to kill Havilland, because Havilland was a threat to him. Who found this assassin, if not Sixsmith?"

Monk felt as if he were on the witness stand. It was more uncomfortable than he had anticipated. It would be impossible to escape with inaccurate or incomplete answers. "Alan Argyll himself, or perhaps Toby," he answered. "Alan has taken great care to account for all his own time before and after Havilland's death, but Toby was several years younger and spent more time on the sites and knew some of the tougher navvies."

"According to whom?" Rathbone said quickly.

Monk smiled, but without pleasure. "According to Sixsmith. But it can be easily verified."

"You'll need to do it," Rathbone warned. "The money came from Argyll, you say?"

"Yes."

"If he says it was for wages, or a new machine, and that Sixsmith misappropriated it, can you prove he's lying?"

Monk felt his muscles tighten defensively. "No, not beyond a doubt."

"Reasonable doubt?"

"I don't know what doubt is reasonable. I'm certain myself."

"Not exactly relevant," Rathbone said dryly. "Why would Argyll want Havilland dead so much that he would be prepared to use Sixsmith to hire an assassin?"

"Knowledge that the tunnels were dangerous and work should be stopped," Monk replied.

"Isn't all such work dangerous? The Fleet sewer collapse was appalling."

"That's cut-and-cover," Monk told him. "Imagine that underground, possibly collapsing at both ends, with water, or worse-gas."

"Is gas worse? I would have thought water would be pretty dreadful."

"The gas would be methane. That's flammable. It would only need one spark and the whole thing would be ablaze. If it came up through the sewers, it could start another Great Fire of London."

Rathbone paled. "Yes, I have the idea, Monk. Why do you think that is anything more than a madman's nightmare? Surely Argyll wouldn't want that any more than Havilland or anyone else. If it were a real danger, he'd stop the work himself. What was he afraid of-that Havilland would frighten the workforce and they'd strike? Why not just bar him from the site? Isn't murder excessive, not to mention dangerous and expensive?"

"If it wasn't the navvies Havilland was going to, but the authorities, that would be different. He couldn't stop that so easily. And even an unfounded fear could close the excavations for enough time to delay the work seriously and cost a great deal of money. To a ruthless man, one perhaps running rather close to the edge of profit and loss, or with an over-large investment, that could be motive for murder."

Rathbone frowned. "But motive is not enough, Monk, which you know as well as I. Why not suppose it was Sixsmith, exactly as it appears to be?"

"Because it was Argyll's wife who sent the letter to her father asking him to be in the stable after midnight," Monk answered decisively. "At Argyll's request."

"And if Argyll says he did not ask her to write it?" Rathbone asked. "You cannot force her to incriminate him. It would be profoundly against her interest."

"Others will swear it is her handwriting."

"You have the letter?"

"I don't. I have the envelope."

"The envelope! For God's sake, Monk! Anything could have been in it! Did anyone see the letter? Is the envelope postmarked?"

Monk felt the argument slipping out of his grasp. "The envelope was hand delivered," he replied levelly. "But it is beyond reasonable doubt that it was the one he received that evening, because he made notes on it in his own hand, and it was in the pocket of the jacket he was wearing. That's where we found it."

"Could it have belonged to another letter sent at an earlier time?"

"There were notes on it relating to events that happened that evening," Monk replied with satisfaction.

"Good. So Mrs. Argyll sent him a note. If she swears it was an invitation to dinner in a week's time, and she is willing to, what have we?"

"A woman prepared to lie to two police officers, under oath."

"To save her husband, her home, her source of income, and her position in society-and thus also her children." Rathbone puckered his mouth into a tight, bleak smile. "Not an unusual phenomenon, Monk. And not one you would find it easy, or popular, to destroy. You would not win the jury's favor with that."

"I want their belief, not their favor!" Monk snapped.

"Juries are driven by emotion as well as reason," Rathbone pointed out. "You're playing a dangerous game. I can see about charging Sixsmith as an accessory, possibly an unknowing one as far as murder is concerned, and hope to draw out enough to implicate Argyll, but you'd have to come up with a lot more than you have so far." His face pinched a little. "It happens sometimes. You can catch everyone but the real culprit. It looks as if Argyll's protected himself pretty well. To reach him you'll have to destroy this man Sixsmith, who may be completely innocent of anything except a fairly usual business bribe. You'll also destroy Argyll's wife, who is doing what any woman would do to protect her children, perhaps even to protect her belief in her husband as a decent man. And she may need that to survive with any kind of sanity."

Monk hesitated. Was it worth it? Should he destroy the slightly tarnished, those culpable only of ordinary human weakness, in order to reach the truly guilty? For what-vengeance? Or to protect future victims?

"You don't have a choice now," Rathbone said quietly. "At least not as far as Sixsmith is concerned. I'll prosecute, by all means, and uncover everything I can. Meanwhile, you find out more about this mysterious assassin. Show who contacted him, if he ever took the second payment, if he knows who employed him. Above all, you need to show what Havilland was going to do that was sufficient to make Argyll want to kill him. So far all you have is an engineer who lost his nerve and became a nuisance. Sane men don't commit murder for that. Give me chapter and verse of what Argyll would lose, and connect it to him, not just to Sixsmith."

Monk stood up. "I'll find it! How long have I?"

"Till it comes to trial? Three weeks."

"Then I'd better start." He moved towards the door.

"Monk!"

He turned back. "Yes?"

"If you're right and it is Argyll, be careful. He's a very powerful man and you work in a dangerous job."

Monk stared at Rathbone with sudden surprise. There was a gentleness in his face he had not expected to see. "I will," he promised. "I have good men around me."