Here's my thought..
Two little lovebirds in what seems a decent, normal (boring, mundane, safe) relationship. Then, through fluke.. perhaps watching the right very naughty film one night.. or maybe coming upon some seedy self-help book that turns out to be an instructional guid to the delicious world of bondage..
These two innocents suddenly find themselves hurtling fast (and at first, possibly reluctant but intrigued) into something they seemed absolutely destined (despite their fighting these growing urges)towards. In fact, this world of theirs is turned upside down in the best way as each discovers not only their inner Dominant and submissive.. but that these beings lurking within are actually their truest selves.
I think such an exploration would be one hell of a wild ride.