By the time Derek arrived at the shed in which his Dromedary biplane was kept, he felt that much of his drowsiness had passed. It was a fair night, although slightly overcast. Occasionally the stars shone through the wide rifts in the vapour. There was little or no wind. "All ready?" he asked of the Sergeant-Mechanic. "All ready, sir," was the reply. By sheer force of habit Daventry tested the controls, and assured himself that the petrol-tank was filled. Then, donning his flying-kit, he clambered into his seat. Along the electrically-lighted ground the biplane ambled, and then rose magnificently into the night air. A moment later and the powerful arc-lamps were switched off, and the countryside beneath the rapidly-climbing 'bus was shrouded in utter darkness. At six thousand feet Derek found that his sense of lassitude had completely vanished. The bracing coldness of the rarefied atmosphere acted more effectually than the best tonic prepared by human agency. More than once he realized that he was singing at the top of his voice, as if trying to outrival the terrific roar of the powerful motors. He was now well above the stratum of clouds. Overhead the stars shone brilliantly. He was alone, rushing through space at a speed of ninety miles an hour. "Goodness only knows why I'm up here," he reiterated. "Anyway, it's a jolly picnic. I'll cut out and see if anything's doing." Accordingly, Daventry shut off the engine and began vol-planing as gently as possible. He listened intently for the roar of a hostile propeller above the swish of the air past struts and tension-wires. "Thought so," he muttered, as he restarted the motor. "Nothin' doin'. I'm on a dud stunt. However, I'll carry on." For the best part of an hour Derek continued his flight, describing huge figures-of-eight in order to keep in touch with the aerodrome. In vain he maintained a sharp look-out for any lurid bursts of flame on the distant horizon that would indicate that the Boche was setting to work, and that the anti-aircraft guns were giving the raiders a hot tonic. He was on the point of discharging his signal-pistol in order to inform the aerodrome that he was about to make a landing when a dark, indistinct mass shot by a hundred feet below him, and then vanished in the darkness. "By Jove! I wonder if that's a Fritz?" ejaculated the young pilot. "I'll try and find out." Almost before the Dromedary began to rock in the eddies in the wake of the mysterious aeroplane Derek swung his 'bus round, banking steeply ere he steadied her on her course. A glance at the altimeter showed him that the height was eight thousand five hundred feet, quite enough manoeuvring space for the work in hand, provided he could find his quarry.