"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?