Tzvetan Todorov, in The Poetics of Prose, argues that the classic
detective story has a dual structure. It ‘contains not one but
two stories: the story of the crime and the story of the investigation.
In their purest form, these two stories have no point in common . .
. . The first story, that of the crime, ends before the second begins.
But what happens to the second? Not much. The characters of the second
story, the story of the investigation, do not act, they learn. Nothing
can happen to them: a rule of the genre postulates the detective's immunity.
We cannot imagine Hercule Poirot or Philo Vance threatened by some danger,
attacked, wounded, even killed. The hundred and fifty pages which separate
the discovery of the crime from the revelation of the killer are devoted
to a slow apprenticeship: we examine clue after clue, lead after lead.
The whodunit thus tends toward a purely geometric architecture. . .
.
‘This second story, the story of the investigation, . . . is often
told by a friend of the detective, who explicitly acknowledges that
he is writing a book; the second story consists, in fact, in explaining
how this very book came to be written . . . . The first [story] -- the
story of the crime -- tells 'what really happened,' whereas the second
-- the story of the investigation -- explains 'how the reader (or the
narrator) has come to know about it….
‘The first, that of the crime, is in fact the story of an absence:
its [salient] characteristic is that it cannot be immediately present
in the book. In other words, the narrator cannot transmit directly the
conversations of the characters who are implicated, nor describe their
actions: to do so, he must necessarily employ the intermediary of another
(or the same) character who will report, in the second story, the words
heard or the actions observed. The status of the second story . . .
[consists in being] a story which has no importance in itself, which
serves only as a mediator between the reader and the story of the crime
. . . . We are concerned then in the whodunit with two stories of which
one is absent but real, the other present but insignificant.’