round all the time; but Finlay hasn't
once taken a taxi."

"I'm afraid he's taking a big risk----" began Colonel Walton. "That
Naylor fellow----" He paused.

Sage nodded.

During the previous ten days Department Z. had learned a great deal
about the comings and goings of Mr. Montagu Naylor of Streatham. It
had become manifest to Sage that he had to do with a man who had
reduced cunning and caution to a fine art. His every act seemed to
have been carefully thought out beforehand, not only in relation to
himself, but to what might grow directly out of it.

During a walk he would sometimes turn suddenly and proceed swiftly in
the direction from which he had come, as if he had forgotten something,
looking keenly at every one he passed. At others he would step into a
shop, where he could be seen keeping a careful watch through the
window. A favourite trick was to walk briskly round a corner, then
stop and look in some shop window with a small mirror held in the palm
of his hand.

From the first Malcolm Sage had realised that the conventional methods
of shadowing a suspect would be useless for his purpose. Those in whom
Department Z. were interested would be old hands at the game, and to
set a single person to watch them would inevitably result in the
discovery of what was afoot. He therefore set at least three men, or
women, to dog the footsteps of the suspect.

These would follow each other at intervals of from twenty-five to a
hundred yards, according to the district in which they were operating.
At a signal that the first in the line was dropping out, the trail
would be taken up by number two, who in turn would relinquish the work
to number three. Sometimes as many as six were allocated to one
shadowing.

This method had the additional advantage of enabling the Department to
assure itself that the watchers were not in turn being watched.

It was no uncommon thing for a suspect to arrange to have himself
shadowed in order to ascertain whether or no there were any one on his
track. This was a favourite device with Mr. Naylor.

For nearly two years Department Z. had been endeavouring to solve the
problem of a secret organisation, with the offshots of which they were
constantly coming into contact. The method this organisation adopted
was one of concentration upon a single object. At one time it would be
at the sailing of vessels from home ports, at another the munitions
output, or again the anti-airc