Of all the names that he could have heard, that is the one he expected the least.

"You are not," Watson says immediately, suddenly heated then catches himself. Oh, he remembers Joan and everything they had in common, and the terrible, stupid parallels between their lives. He knows. That doesn't make this any easier to deal with. 'Mycroft Holmes' is, ultimately, not a common name. "No, forgive me. I knew a Mycroft Holmes, and he was not -- please. Humour me. Do you happen to have a brother named Sherlock?"

He's not at all sure what he wants the answer to be.