To the murder of Professor Grimaud, and later the equally incredible crime in Cagliostro Street, many fantastic terms could be applied – with reason. Those of Dr Fell’s friends who like impossible situations will not find in his casebook any puzzle more baffling or more terrifying. Thus: two murders were committed, in such fashion that the murderer must not only have been invisible, but lighter than air. According to the evidence, this person killed his first victim and literally disappeared. Again according to the evidence, he killed his second victim in the middle of an empty street, with watchers at either end; yet not a soul saw him, and no footprint appeared in the snow. (Location 31)
