The princess is sent away to a foreign nation for diplomatic reasons, just for a year or two. To stop a war between that nation and her fathers.
Unfortunately the king is unmarried and she is worried the diplomatic mission will become more. So she hatches a plan...
Dress her favorite night as a foreign lord she is already betrothed to, and have him play the part to fend off advances. It's only a year or two it should be fine.
Kisses in public, shared chambers and no one else they know, quickly makes the princess and the knight who already had unspoken crushes on each other the other didn't know about because it could never work build into an overwhelming tension.
The kisses start coming too easily. A month in and she suggests he doesn't need to sleep on the floor. Tension building and building... It will all go back to normal when they return home but why shouldn't they enjoy it here where no one knows them.
Oh. Oh no. How could it ever go back to how it once was? How could I possibly return to that reality? Where the face I've grown to love seeing each morn is once more hidden behind steel? The lips that once poured sweet nothings against my ear each eve speak of nothing but sworn fealty? The warm embrace that once enveloped me reduced to nothing more than light brushes of the hand? It will be my ruin.