It was half-past eleven on the night of Wednesday, April 14th, when the well-known red limousine of Mr. Maverick Narkom, superintendent of Scotland Yard, came abruptly to the head of Mulberry Lane, which, as you may possibly know, is a narrow road skirting one of the loneliest and wildest portions of Wimbledon Common.Lennard, the chauffeur, put on the brake with such suddenness that the car seemed actually to rise from the earth, performed a sort of buzzing and snorting semicircle, and all but collided with the rear wall of Wuthering Grange before coming to a halt in the narrow road space which lay between that wall and the tree-fringed edge of the great Common.Under ordinary circumstances one might as soon have expected to run foul of a specimen of the great auk rearing a family in St. Paul's churchyard, as to find Mr. Narkom's limousine in the neighbourhood of Mulberry Lane at any hour of the day or the night throughout the whole cycle of the year.For a reason which will be made clear in the course of events, however, the superintendent had been persuaded to go considerably out of his way before returning to town after mingling duty with pleasure in taking part in the festivities attendant upon the coming of age of his friend Sir Philip Clavering's son and heir, and, incidentally, in seeing, too, that Petrie and Hammond, two of his sergeants, kept a watchful eye upon the famous Clavering service of gold plate which had been brought out of the bank vault for the occasion.All three were sitting serenely back among the cushions of the limousine at the period when Lennard brought it to this abrupt and startling halt, the result of which was to fairly jerk them out of their seats and send them sprawling over one another in a struggling heap.There was a moment of something like absolute confusion, for mist and darkness enveloped both the road and the Common, and none of the three could see anything from the windows of the car which might decide whether they had collided with some obstruction or were hovering upon the brink of some dangerous and unexpected pitfall.Nor were their fears lessened by perceiving-through the glass screen-that Lennard had started up from his seat, and, with a hastily produced electric torch in one upraised hand, was leaning forward and wildly endeavouring to discern something through the all-enfolding mist. Mr. Narkom hastily unlatched the door and leaned out."What is it? What's gone wrong?" he inquired in the sharp staccato of excitement. "Anything amiss?""Lord, yessir! I heard a shot and a cry. A pistol shot ... and a police whistle ... and a cry of murder, sir. Up the lane ahead of us!" began Lennard, in a quaking voice; then he uttered a cry of fright, for, of a sudden, the darkness was riven by the screaming note of a police whistle-of two police whistles in fact: shrilling appeal and answer far up the lonely lane.Hard on this came a man's voice shouting: "Head him off there, whoever you are! Don't let him get by you. Look sharp! He's making for the railway arch!""All right, mate. I'm here!" another male voice flung back. "He won't get past me, the blighter!"Instantly there struck out the swift-measured sound of heavily shod feet racing at top speed up the mist-shrouded lane, and rapidly increasing the distance between the unseen runner and the standing limousine.No need to tell either Narkom or his men that the man whose steps they heard was a constable, for there is a distinctive note, to ears that are trained, rung out by the heavy, cumbersome boots which folly accords to the British policeman.