Mary Trevert set her mouth in an obstinate line.

"No!" she affirmed uncompromisingly. "The police believe that, as the result of a scene between Hartley and Robin, Hartley killed himself.

Until I've found out for certain whether this letter implicates Robin or not, I sha'n't give it to the police ..."

"But, if Greve really had nothing to do with this shocking tragedy, the police can very easily clear him. Surely they are the best judges of his guilt ..."

Again a touch of warm colour suffused the girl's cheeks. Euan MacTavish remarked it and looked at her wistfully.

"Well, well," he observed gently, "perhaps they're not, after all!"

The girl looked up at him.

"Euan, dear," she said impulsively, "I knew you'd understand. Robin and Hartley may have had a row, but it was nothing worse. Robin is incapable of having threatened--blackmailed--Hartley, as the police seem to imagine. I am greatly upset by it all; I can't see things clear at all; but I'm determined not to give the police a weapon like this to use against Robin until I know whether it is sharp or blunt, until I have found out what bearing, if any, this letter had on Hartley Parrish's death ..."

Euan MacTavish leant back in his chair and said nothing. He finished his cigarette, pitched the b.u.t.t into the fender, and turned to Mary. He asked her to let him see the letter again. Once more he read it over.

Then, handing it back to her, he said:

"It's all so simple-looking that there may well be something behind it.

But, if you do go to Holland, how are you going to set about your enquiries?"

"That's where you can help me, Euan, dear," answered the girl. "I want to find somebody at Rotterdam who will help me to make some confidential enquiries about this firm. Do you know any one? An Englishman would be best, of course ..."

But Euan MacTavish was halfway to the door.

"Wait there," he commanded, "till I telephone the one man in the world who can help us."

He vanished into the hall where Mary heard him at the instrument.

"We are going round to the Albany," he said, "to see my friend, Ernest Dulkinghorn, of the War Office. He can help us if any one can. But, Mary, you must promise me one thing before we go ... you must agree to do what old Ernest tells you. You needn't be afraid. He is the most unconventional of men, capable of even approving this madcap scheme of yours!"

"I agree," said Mary, "but how you waste time, Euan! We could have been at the Albany by this time!"

In a first-floor oak-panelled suite at the Albany, overlooking the covered walk that runs from Piccadilly to Burlington Gardens, they found an excessively fair, loose-limbed man whose air of rather helpless timidity was heightened by a pair of large tortoise-sh.e.l.l spectacles. He appeared excessively embarra.s.sed at the sight of MacTavish's extremely good-looking companion.

"You never told me you were bringing a lady, Euan," he said reproachfully, "or I should have attempted to have made myself more presentable."

He looked down at his old flannel suit and made an apologetic gesture which took in the table littered with books and papers and the sofa on which lay a number of heavy tomes with marked slips sticking out between the pages.

"I am working at a code," he explained.

"Ernest here," said MacTavish, turning to Mary, "is the code king. Your pals in the Intelligence tell me, Ernest, that you've never been beaten by a code ..."

The fair man laughed nervously.

"They've been pullin' your leg, Euan," he said.

"Don't you believe him, Mary," retorted her cousin. "This is the man who probably did more than any one man to beat the Boche. Whenever the brother Hun changed his code, Brother Ernest was called in and he produced a key in one, two, three!..."

"What rot you talk, Euan!" said Dulkinghorn. "Working out a code is a combination of mathematics, perseverance, and inspiration with a good slice of luck thrown in! But isn't Miss Trevert going to sit down?"

He cleared the sofa with a sweep of his arm which sent the books flying on to the floor.

"Ernest," said MacTavish, "I want you to give Miss Trevert here a letter to some reliable fellow in Rotterdam who can a.s.sist her in making a few enquiries of a very delicate nature!"

"What sort of enquiries?" asked Dulkinghorn bluntly.

"About a firm called Elias van der Spyck," replied Euan.

"Of Rotterdam?" enquired the other sharply.

"That's right! Do you know them?"

"I've heard the name. They do a big business. But hadn't Miss Trevert better tell her story herself?"

Mary told him of the death of Hartley Parrish and of the letter she had found upon his desk. She said nothing of the part played by Robin Greve.

"Hmph!" said Dulkinghorn. "You think it might be blackmail, eh? Well, well, it might be. Have you got this letter about you? Hand it over and let's have a look at it."

His nervous manner had vanished. His face seemed to take on a much keener expression. He took the letter from Mary and read it through.

Then he crossed the room to a wall cupboard which he unlocked with a key on a chain, produced a small tray on which stood a number of small bottles, some paint-brushes and pens, and several little open dishes such as are used for developing photographs. He bore the tray to the table, cleared a s.p.a.ce on a corner by knocking a pile of books and papers on the floor, and set it down.

"Just poke the fire!" he said to Euan.

From a drawer in the table he produced a board on which he pinned down the letter with a drawing-pin at each corner. Then he dipped a paint-brush into one of the bottles and carefully painted the whole surface of the sheet with some invisible fluid.

"So!" he said, "we'll leave that to dry and see if we can find out any little secrets, eh? That little tray'll do the trick if there's any monkey business to this letter of yours, Miss Trevert. That'll do the trick, eh, what?"

He paced the room as he talked, not waiting for an answer, but running on as though he were soliloquizing. Presently he turned and swooped down on the board.

"Nothing," he e.j.a.c.u.l.a.t.ed. "Now for the acids!"

With a little piece of sponge he carefully wiped the surface of the letter and painted it again with a substance from another bottle.

"Just hold that to the fire, would you, Euan?" he said, and gave MacTavish the board. He resumed his pacing, but this time he hummed in the most unmelodious voice imaginable:

She was bright as a b.u.t.terfly, as fair as a queen, Was pretty little Polly Perkins, of Paddington Green.

"It's dry!"

MacTavish's voice broke in upon the pacing and the discordant song.

"Well?"

Dulkinghorn snapped out the question.

"No result!" said Euan. He handed him the board.

Dulkinghorn cast a glance at it, swiftly removed the letter, held it for an instant up to the electric light, fingered the paper for a moment, and handed the letter back to Mary.