The Fatal Mistake of Mr. Quinn

London. 1928. A few months after his first successful case, writer Marius Quinn receives alarming news from his girlfriend Lady Isabella. Their childhood friends, the Hinton cousins, have started receiving anonymous letters with threats, but the local constable considers it nothing more than a prank. Therefore, Isabella asks Marius to come to their hometown, which he hasn't visited for many years after the mysterious disappearance of his father. He has no choice but to agree. Loading his entire family and beloved dog Percy into the car, Marius sets off for the village. But it turns out to be too late. The next morning, he finds his friend's body with a knife in his heart. Could one of the sweet and slightly eccentric neighbors really be capable of murder? And what old secret does the criminal hint at in the letters? Marius and Bella have until the end of the week to find the murderer; otherwise, he will exterminate the remaining Hintons.