A whodunit story is a type of mystery fiction in which the reader is presented with a crime and must deduce the identity of the perpetrator from clues presented throughout the story. An ALIBI is an excuse or proof that someone was elsewhere when a crime was committed, and is often used by suspects to prove their innocence. In a whodunit story, alibis are often used by suspects to try to prove their innocence, and it is up to the reader to determine if the ALIBI is valid or not.