Disclaimer: Castle isn't mine.
Detective Katherine Beckett had a secret lover.
They had to be stealthy with their trysts, holding them in the wee hours of the morning when no one was around the precinct.
Beckett knew that the other detectives at the station would enjoy ribbing her far too much if their affair were ever discovered. And Castle… God, he'd be insufferable.
So, after she and her secret lover had parted ways, she made sure that no traces remained of their passion. The only thing that lingered was on her lips – the taste of their parting kiss. By the time the first detective came straggling in, Beckett was sitting composedly at her desk, doing paperwork, a mug of plain precinct coffee sitting by her hand.
No could ever suspect that she harbored a passionate love for Castle's girly coffee.
A/N: The purpose of the Forbidden Romance challenge is, in Shailene Cullen's words, "to come up with the most off-the-wall, could-never-happen couple you can think of". But let's face it:
A. This love isn't really forbidden. The only obstacle to the affair is Beckett herself.
B. This affair has already been consummated multiple times over; it's canon. ;-)
On the other hand, Beckett/Castle's coffee isn't a traditional pairing in fanfic… which is why this is a sort-of entry to the challenge.