There was a crooked man
and he went a crooked mile.
He found a crooked sixpence
beside a crooked stile.
He had a crooked cat
which caught a crooked mouse,
And they all lived together
in a little crooked house.
Crooked House is a stand-alone novel. I.e. it does not feature any of Christie's established sleuths (Marple, Poirot, Tommy & Tuppence, etc.).
The story tells of a young couple, Charles and Sophia, who decide to postpone any decisions on getting married until after the war. Once the time has come the engagement is again interrupted by a death in Sophia's family.
From then on, trust is put to the test and motives are questioned. Everyone is a suspect and it is left to the couple to discover whom they can believe, or if they can at all.
What is interesting with this story is the finding out who the murderer is almost takes a backseat to getting to know the characters of Sophia's relatives. What an interesting bunch of eccentrics! They are all suffering from a past dominated by the misanthropic grandfather who claimed to have killed two people in his youth.
It's a marvellously dark story and it is easy to see why it was one of Dame Agatha's favourite mysteries.