Cosmic Saboteur by Frank M. Robinson

It was eight o'clock Thursday evening when Stan stepped out of a faintly glowing circle of black light in a small alley off the Rue Pigalle in Paris. He calmly lit a cigarette and walked down the street to a small cafe.

It was bigger on the inside than it looked from the street. A long, low-ceilinged room with a tiny platform and a small band, almost hidden by the cigarette smoke, at the far end. Tanner and Reynolds, one of Stan's lieutenants, were seated at a small table along the side, earnestly talking to a frightened little man with an old-fashioned walrus moustache.