Poirot said, "This case is, in my opinion, one of the most intriguing I have ever encountered. There is nothing to see to begin with. There are four people, and one of them is certainly the perpetrator, but which one? Is there anything that can shed light on the matter? At the crime scene—no, there is no physical evidence, no fingerprints, no papers or documents that implicate anyone in a crime. There are only the four people themselves." Indeed, this story exemplifies the concept of a "well-crafted crime" at its finest. It is the kind of case in which Christie excelled, and it is itself the ultimate test of the reader's wit and intelligence. Four people are playing cards, and at the height of their game, their host—who was sitting outside—is murdered. He could only have been killed by one of these four while they were playing cards. According to the sequence of events, each of the four had the opportunity to commit the crime. Each had committed a crime before, to varying degrees, and each had the capacity to commit another.