“That’s the way Sir Henry led off with his explanation.  Of course he had all that Scotland Yard knew about criminal groups to start him right.  It was a good deal to have the identity of the criminal agents selected out; but I didn’t see how he was going to manage to explain the mystery from the evidence.  I was wild to hear him.  Mr. Meadows was quite as interested, I thought, although he didn’t say a word.

“Sir Henry nodded, as though he took the American’s confirmation as a thing that followed. `We are at the scene,’ he said, `of one of the most treacherous acts of all criminal drama.  I mean the “doing in,” as our criminals call it, of the unprofessional accomplice.  It’s a regulation piece of business with the hard-and-fast criminal organizations of the Continent, like the Nervi of Marseilles, or the Lecca of Paris.

“`They take in a house servant, a shopkeeper’s watchman, or a bank guard to help them in some big haul.  Then they lure him into some abandoned house, under a pretense of dividing up the booty, and there put him out of the way.  That’s what’s happened here.  It’s a common plan with these criminal groups, and clever of them.  The picked-up accomplice would be sure to let the thing out.  For safety the professionals must “do him in” at once, straight away after the big job, as a part of what the barrister chaps call the res gestae.’

“Sir Henry went on nodding at us and drumming the palm of his hand on the edge of the table.

“`This thing happens all the time,’ he said, `all about, where professional criminals are at work.  It accounts for a lot of mysteries that the police cannot make head or tail of, like this one, for example.  Without our knowledge of this sinister custom, one could not begin or end with an affair like this.

“`But it’s simple when one has the cue — it’s immensely simple.  We know exactly what happened and the sort of crooks that were about the business.  The barefoot prints show the Continental group.  That’s the trick of Southern Europe to go in barefoot behind a man to kill him.’

“Sir Henry jarred the whole table with his big hand.  The surface of the table was covered with powdered chalk that the baronet had dusted over it in the hope of developing criminal finger prints.  Now under the drumming of his palm the particles of white dust whirled like microscopic elfin dancers.

“`The thing’s clear as daylight,’ he went on:  `One of the professional group brought the accomplice down here to divide the booty.  He broke the door in.  They sat down here at this table with the lighted candle as you see it.  And while the stuff was being sorted out, another of the band slipped in behind the man and killed him.

“`They started to carry the body out.  Millson chanced by.  They got in a funk and rushed the thing.  Of course they had a motor down the road, and equally of course it was no trick to whisk the body out of the neighborhood.’