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 tonight.’

‘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 tonight 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 today. 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.’