A man was gunned down in his flat by a burglar, who then ransacked the flat.
The case was placed in the capable hands of Inspector Gilbert Bodwin of Scotland Yard.
Bodwin’s investigation revealed that one man had planned the crime, another had carried it out, and a third had acted as lookout and fourth one was somehow present unaware of plan.
Bodwin discussed the case at length one evening over dinner at his club with an old friend, Thomas P. Stanwick, the amateur logician visiting from America.
“It’s quite a case,” Stanwick remarked. “Any suspects?”
Bodwin sliced his roast beef. “Yes indeed. Four. We have conclusive evidence that three of those four were responsible for the crime.”
“Really! That’s remarkable progress. What about the fourth?”
“He had no prior knowledge of the crime and is completely innocent. The problem is that we’re not sure which of the four are the planner, the gunman, the lookout, and the innocent bystander.”
“I see.” Stanwick took more Yorkshire pudding. “What do you know about them at this point?”
“Well, the names of the four are Merrick, Cross, Llewellyn, and Halifax. Halifax and Cross play golf together every Saturday. They’re an odd pair! Halifax can’t drive, and Cross has been out of Dartmoor Prison for only a year.”
“What was he in for?”
“Forgery. We know that Merrick and Halifax kept the flat under the surveillance for several days just before the day the crime was committed, the 17th. Llewellyn and Merrick, with their wives, had dinner together on the Strand on the 12th.”
“An interested compilation,” said Stanwick, “but hardly conclusive. Is that all of it?”
“Not quite. We know that the gunman spent the week before the crime in Edinburgh, and that the innocent bystander was acquainted with the planner and the gunman, but not with the lookout.”
"That’s very helpful,” said Stanwick with a smile. “Bodwin, your case is complete!”

WHO WERE THE PLANNER, THE GUNMAN, AND THE LOOKOUT?