Set in 1870s Oxford, “Younger Sherlock” imagines Sherlock Holmes earlier than Baker Road legend calcified round him — a 19-year-old hothead, uncooked, undisciplined, and already in bother, pulled into his first main case when a homicide and a bigger worldwide conspiracy threaten his freedom and set him on the highway towards the detective he’ll turn out to be. It’s a wonderfully serviceable origin-story setup, the form of premise that provides a collection room to construct character, rivalry, and fable.