Hollow man, the

The murderer of Dr Grimaud walked through a locked door, shot his victim, and vanished. He killed his second victim in the middle of an empty street, with watchers at each end, yet nobody saw him, and he left no footprint in the snow. It is left to the gargantuan Dr Fell, with his rumbling laugh and his bandit's moustache, to solve this most famous and taxing of locked-room mysteries.