What is the theme in Poirot Investigates by Agatha Christie?

Crime is the general theme of all the stories in this collection. Criminals are portrayed mostly with vices although many display intelligence. Poirot only deals with the most difficult cases, being disdainful about some crimes that are too easy to solve. Mystery surrounds every crime that sometimes appears to be straightforward and obvious. In most cases the crime is solved and if the criminals are not caught, they face natural justice. Crime seems to be ever present as it reoccurs almost anywhere Poirot and Hastings appear. Whether they are sick or just want to go for holidays, they get involved, sometimes reluctantly, in finding a murderer. Poirot becomes passionate about some of the cases, undertaking some voluntarily. He does crime detection for money, although sometimes admits that crime can be more profitable. Crime is committed by poor people but it is present also among the rich people who are not immune to vice.