John and Mary invite Sherlock to join them in a threesome. Sherlock isn't very comfortable with the idea, but he agrees because John wants it and he knows that it's his only chance to be with John. The problem is that sex does alarm Sherlock, and he's only able to be intimate with people he trusts 100%. John qualifies, of course, but Mary, the person who shot him in cold blood? Sherlock had stifled all feelings of distrust towards her for John's sake, but once they're all naked in bed and Sherlock's terrified, it all comes pouring out. Sherlock has a bad panic attack and John has to try to calm him down.