"In that case," said Tommy, opening the side cupboard, and surveying his rows of books with an affectionate eye. "Our role is easy to select. We are the brothers Okewood! And I am Desmond," he added firmly.

Tuppence shrugged her shoulders.

"All right. Have it your own way. I'd just as soon be Francis. Francis was much the more intelligent of the two. Desmond always gets into a mess, and Francis turns up as the gardener or something in the nick of time, and saves the situation."

"Ah!" said Tommy, "but I shall be a super Desmond! When I arrive at The Larches-"

Tuppence interrupted him unceremoniously.

"You're not going to Hampstead to-night?"

`'Why not?"

"Walk into a trap with your eyes shut!"

"No, my dear girl, walk into a trap with my eyes open. There's a lot of difference. I think our friend Dr. Bower will get a little surprise."

"I don't like it," said Tuppence. "You know what happens when Desmond disobeys the Chief's orders, and acts on his own. Our orders were quite clear. To send on the letters at once and to report immediately on anything that happened."

"You've not got it quite right," said Tommy. "We were to report immediately if anyone came in and mentioned the number 16. Nobody has."

"That's a quibble," said Tuppence.

"It's no good. I've got a fancy for playing a lone hand. My dear old Tuppence, I shall be all right. I shall go armed to the teeth. The essence of the whole thing is that I shall be on my guard and they won't know it. The Chief will be patting me on the back for a good night's work."

"Well," said Tuppence. "I don't like it. That man's as strong as a gorilla."

"Ah!" said Tommy, "but think of my blue-nosed automatic."

The door of the outer office opened and Albert appeared. Closing the door behind him, he approached them with an envelope in his hand.

"A gentleman to see you," said Albert. "When I began the usual stunt of saying you were engaged with Scotland Yard, he told me he knew all about that. Said he came from Scotland Yard himself! And he wrote something on a card and stuck it up in this envelope."

Tommy took the envelope and opened it. As he read the card, a grin passed across his face.

"The gentleman was amusing himself at your expense by speaking the truth, Albert," he remarked. "Show him in."

He tossed the card to Tuppence. It bore the name Detective Inspector Dymchurch, and across it was scrawled in pencil-"A friend of Marriot's."

In another minute the Scotland Yard detective was entering the inner office. In appearance, Inspector Dymchurch was of the same type as Inspector Marriot, short and thick set, with shrewd eyes.

"Good afternoon," said the detective breezily. "Marriot's away in South Wales, but before he went, he asked me to keep an eye on you two, and on this place in general. Oh! bless you, sir," he went on, as Tommy seemed about to interrupt him, "we know all about it. It's not our department, and we don't interfere. But somebody's got wise lately to the fact that all is not what it seems. You've had a gentleman here this afternoon. I don't know what he called himself, and I don't know what his real name is, but I know just a little about him. Enough to want to know more. Am I right in assuming that he made a date with you for some particular spot this evening?"

"Quite right."

"I thought as much. 16 Westerham Road, Finsbury Park? Was that it?"

"You're wrong there," said Tommy with a smile. "Dead wrong. The Larches, Hampstead."

Dymchurch seemed honestly taken aback. Clearly he had not expected this.

"I don't understand it," he muttered. "It must be a new layout. The Larches, Hampstead, you said?"

"Yes. I'm to meet him there at eleven o'clock to-night."

"Don't you do it, sir."

"There!" burst from Tuppence.

Tommy flushed.

"If you think, Inspector-" he began heatedly.

But the Inspector raised a soothing hand.

"I'll tell you what I think, Mr. Blunt. The place you want to be at eleven o'clock to-night is here in this office."

"What?" cried Tuppence, astonished.

"Here in this office. Never mind how I know-departments overlap sometimes-but you got one of those famous "Blue" letters to-day. Old what's his name is after that. He lures you up to Hampstead, makes quite sure of your being out of the way, and steps in here at night when all the building is empty and quiet to have a good search round at his leisure."

"But why should he think the letter would be here? He'd know I should have it on me or else have passed it on."

"Begging your pardon, sir, that's just what he wouldn't know. He may have tumbled to the fact that you're not the original Mr. Blunt, but he probably thinks that you're a bona fide gentleman who's bought the business. In that case, the letter would be all in the way of regular business and would be filed as such."

"I see," said Tuppence.

"And that's just what we've got to let him think. We'll catch him red handed here to-night."

"So that's the plan, is it?"

"Yes. It's the chance of a lifetime. Now, let me see, what's the time? Six o'clock. What time do you usually leave here, sir?"

"About six."

"You must seem to leave the place as usual. Actually we'll sneak back to it as soon as possible. I don't believe they'll come here till about eleven, but of course they might. If you'll excuse me, I'll just go and take a look round outside and see if I can make out anyone watching the place."

Dymchurch departed, and Tommy began an argument with Tuppence.

It lasted some time and was heated and acrimonious. In the end Tuppence suddenly capitulated.

"All right," she said. "I give in. I'll go home, and sit there like a good little girl whilst you tackle crooks and hob nob with detectives-but you wait, young man. I'll be even with you yet for keeping me out of the fun."

Dymchurch returned at that moment.

"Coast seems clear enough," he said. "But you can't tell. Better seem to leave in the usual manner. They won't go on watching the place once you've gone."

Tommy called Albert, and gave him instructions to lock up.

Then the four of them made their way to the garage near by where the car was usually left. Tuppence drove and Albert sat beside her. Tommy and the detective sat behind.

Presently they were held up by a block in the traffic. Tuppence looked over her shoulder and nodded. Tommy and the detective opened the right hand door, and stepped out into the middle of Oxford Street. In a minute or two Tuppence drove on.