“The most fantastic definitions are suitable for the murder of Professor Grimaud, and later for the equally incredible crime on Cagliostro Street. Those of Dr. Fell's friends who enjoy a mystery couldn't find a creepier story in their notebooks. So: two murders have been committed, and in such a way that the killer must not only be invisible, but also lighter than air. According to witness testimony, this killer dealt with his victim and disappeared. Then he committed a second murder in the middle of a deserted street, at both ends of which there were passers-by, but not a single living soul saw him, and there were no prints of his feet left in the snow.”