Cervera and her companion were just mounting the steps of an imposing stone residence, entirely separate from its neighbors, and their carriage was driving rapidly away.

Nick waited until the couple had entered the house, then he crossed to the gloom of a doorway on the opposite side and had a look at the dwelling.

From basement to roof there was no sign of a light.  Even the hall appeared to be in darkness, and Nick waited and watched for several minutes, expecting to see at least one of the rooms lighted.

Not a glimmer or gleam, however, appeared from any quarter.

“H’m!” he presently muttered, a little perplexed.  “Either they are remaining in darkness, or else they have all of those windows heavily curtained.  If the latter is the case, I must discover for what reason.

“Possibly they are entirely alone in there, and have gone to some room at the rear of the house.  Or maybe they have suspected an espionage, and are now watching from the gloom of one of those front windows.  I’ll fool them if that is so, and will also have a look at the rear of the house.  There is something out of the ordinary here, that’s certain.”

Keeping well in the gloom of the block of dwellings near by, Nick retraced his steps to the corner, then crossed the street and presently approached a paved driveway leading to a small stable at the rear of the suspected house.

The high gate, composed of sharp iron pickets, was securely closed and locked; so Nick returned to an alley which he had just passed, and which ran back of a block of dwellings fronting on the avenue where he had left the cab.

Stealing into the alley, Nick quickly scaled the high, wooden fence, crossed two adjoining back yards, and thus reached a wall near the stable mentioned.

To mount the wall and drop back of the stable was equally feasible, and Nick then had the rear of Cervera’s dwelling plainly in view.

Then his searching gaze was rewarded.  One of the rear rooms was brightly lighted, with only the lace draperies at the two windows preventing observation from outside.

“Evidently a rear sitting room, or library,” thought Nick, calculating the arrangement of the house.  “I will at least learn who is in there.”

He listened briefly for any sound in or about the stable, then stole quickly across the gloomy, paved yard and approached the house.

The windows of the lighted room were two feet or more above his head; but having reached a position just below one of them, he sprang up and seized the stone coping outside, and drew himself up to peer into the room.

Then, just as his head rose into the glow of light from within, clearly revealing his location, Nick heard a sound the deadly nature of which he instantly recognized.

Ping!

It was the short, sharp, peculiar song of a flying bullet—­once heard, always remembered.