Are in a D/s relationship and it's very happy. But one day, someone who has no right to, forces Sherlock down into subdrop in a non sexual way.

It goes unnoticed by John and Lestrade and Sherlock is too deep down, he isn't mentally capable of reaching out for help. Sherlocks first instincts are to hide anything wrong.

Then the three get together for a scene which is already too much for Sherlock and they get halfway through before John stops the scene for medical reasons on behalf of Sherlock.

Ok, take it from there.