Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'
7. The Three Garridebs
One of the cases that Eurus forces the Baker Street Boys to solve is the mystery of who killed a man named Evans. She has narrowed it down to the three Garrideb brothers - Howard, Alex and Nathan - but they have to guess which is the murderer.
The names of the key figures in this mystery are taken from Doyle's The Three Garridebs, though the facts of the case are completely different. The villain in the original story, for instance, is "Killer" Evans - a serial killer escaped from the States.
It is fitting that this story is referenced in this episode as it contains the one moment in the canon when Holmes openly admits how much he cares for Watson, when the good doctor gets shot. Though this exact moment isn't recreated, Sherlock's love for John is definitely made plain here.