John's in a relationship with Mary and he's seeing Sherlock on the side. Sherlock believes himself to be a master of disguise and convinces John to go to a club/restaurant/somewhere public on a date with him while in elaborate disguise so they can be as openly affectionate with each other as they please.
However, someone with a keen eye recognises them (Irene? Sally?) and this someone knows Mary, likes Mary quite a bit and is fairly disturbed that John would dare do something like this to Mary. Though, Irene (or Sally) says she could keep quiet if she's paid a certain substantial sum of money or requests favours or something else (Sherlock jumps at the opportunity to keep Irene (or Sally) quiet for John's sake, while John wallows in pity).
Irene or Sally eventually reveal to Mary John's unfaithfulness. Mary leaves John, and Sherlock/John get together. And I would love a bonus of Irene/Mary or Sally/Mary.
Infidelity + blackmail. S/J + Irene (or Sally)
However, someone with a keen eye recognises them (Irene? Sally?) and this someone knows Mary, likes Mary quite a bit and is fairly disturbed that John would dare do something like this to Mary. Though, Irene (or Sally) says she could keep quiet if she's paid a certain substantial sum of money or requests favours or something else (Sherlock jumps at the opportunity to keep Irene (or Sally) quiet for John's sake, while John wallows in pity).
Irene or Sally eventually reveal to Mary John's unfaithfulness. Mary leaves John, and Sherlock/John get together. And I would love a bonus of Irene/Mary or Sally/Mary.