AU where Sherlock's father is from a wealthy high standing family (duke, lord, etc) but Sherlock's father left Sherlock's mother and him and his brother when he was very young (Sherlock and Mycroft go by their mother's maiden name). He isn't aware of his father's heritage until he's called onto a case involving his father, whom he hasn't seen since he was a young boy.

+if Scotland Yarders were there when Sherlock sees his dad.