"You cannot kill me," the spy shouted.  "Your execution squad has just shot me, yet here I stand, as much alive as ever.  There are many more like me in Germany.  Bullets cannot kill them!"  And G-8, listening, knew this strange man spoke the truth—in part at least.  He had already met a Boche staffel whom bullets could not injure!  Who were these invincible pilots?  How could G-8 and his aces dare hope to conquer an enemy that could not die?