“God!” cried Angus involuntarily, “the Invisible Man!”

Without another word he turned and dashed up the stairs, with Flambeau following; but Father Brown still stood looking about him in the snow-clad street as if he had lost interest in his query.

Flambeau was plainly in a mood to break down the door with his big shoulders; but the Scotchman, with more reason, if less intuition, fumbled about on the frame of the door till he found the invisible button; and the door swung slowly open.

It showed substantially the same serried interior; the hall had grown darker, though it was still struck here and there with the last crimson shafts of sunset, and one or two of the headless machines had been moved from their places for this or that purpose, and stood here and there about the twilit place.  The green and red of their coats were all darkened in the dusk; and their likeness to human shapes slightly increased by their very shapelessness.  But in the middle of them all, exactly where the paper with the red ink had lain, there lay something that looked like red ink spilt out of its bottle.  But it was not red ink.

With a French combination of reason and violence Flambeau simply said “Murder!” and, plunging into the flat, had explored, every corner and cupboard of it in five minutes.  But if he expected to find a corpse he found none.  Isidore Smythe was not in the place, either dead or alive.  After the most tearing search the two men met each other in the outer hall, with streaming faces and staring eyes.  “My friend,” said Flambeau, talking French in his excitement, “not only is your murderer invisible, but he makes invisible also the murdered man.”

Angus looked round at the dim room full of dummies, and in some Celtic corner of his Scotch soul a shudder started.  One of the life-size dolls stood immediately overshadowing the blood stain, summoned, perhaps, by the slain man an instant before he fell.  One of the high-shouldered hooks that served the thing for arms, was a little lifted, and Angus had suddenly the horrid fancy that poor Smythe’s own iron child had struck him down.  Matter had rebelled, and these machines had killed their master.  But even so, what had they done with him?

“Eaten him?” said the nightmare at his ear; and he sickened for an instant at the idea of rent, human remains absorbed and crushed into all that acephalous clockwork.

He recovered his mental health by an emphatic effort, and said to Flambeau, “Well, there it is.  The poor fellow has evaporated like a cloud and left a red streak on the floor.  The tale does not belong to this world.”

“There is only one thing to be done,” said Flambeau, “whether it belongs to this world or the other.  I must go down and talk to my friend.”

They descended, passing the man with the pail, who again asseverated that he had let no intruder pass, down to the commissionaire and the hovering chestnut man, who rigidly reasserted their own watchfulness.  But when Angus looked round for his fourth confirmation he could not see it, and called out with some nervousness, “Where is the policeman?”