The four 'friends' of the title could not really be considered friends — they would be better described as uneasy co-vacationers, each of whom carries a secret enmity within his heart.
These men are all fairly well up the social scale and sufficiently wealthy that they can spend their New Year break at the Magnifico, putatively the most expensive and luxurious hotel in England. Yet each is guilty of having committed a crime, at least in terms of the laws which applied at the time, although their transgressions remain secret, or known only to a few. This has left two of the men vulnerable, and all of the men dangerous.
But the aspect I found most intriguing was this often-encountered idea, at least in the older Penguins, that the optimal solution to the affair will be the one in which none of the men need suffer the consequences of their irregular actions. It will be considered ideal, should no one end up murdered, if everything can be set to right without the involvement of police or lawyers, and without anyone ever finding out what they have done. Each man's reputation will then be left unsullied, and his family, friends and business associates will be shielded from the shame which pertains to knowing someone who has behaved inappropriately. I always wonder, when I encounter this attitude in an old Penguin, if the misdemeanours and transgressions of the lower classes were viewed in quite the same way.