Somewhere during an overnight stay (somewhere they've been before Martin joined MJN), Arthur and Douglas both head off for some entertainment (not together, though), leaving Martin behind on his own. Bored, he walks around town and ends up in a club he hadn't realised was a s/m club. Imagine his surprise when he finds his first officer on his knees in front of a total stranger, obediently following every order he's given!

Martin watches without Douglas noticing him, he watches and learns. And the next time they have to share a room, Martin decides that it's time Douglas finally showed him some respect, and he now knows how to make him do it.