Sim, é.
Cuidado, spoilers com relação ao conceito do episódio neste trecho da resenha do The Verge :
This isn’t a true alternate reality, though: having had his season-ending suicidal mission cancelled, the present-day Sherlock has taken a potent — and potentially lethal — drug cocktail. It’s heightening an elaborate fantasy within his "mind palace," and it revolves around an old case that he’s using to solve the mystery of his nemesis Moriarty’s (Andrew Scott) apparent return.