Catalyst is from my own mind but I have to thank something I read on Gritie's Shippers site and the commentary from the episode Memento Mori for ideas. CC, 1013 and Fox own the characters but the rest is all mine.

Prologue – Plausible deniability.

Skinner's office, Monday Morning

'There seems to be some discrepancies concerning your expenses for the last couple of field assignments.' Walter Skinner rebuked the agents before him.

Mulder looked at Scully, in wonderment. Scully, after all, had both written up and handed in the reports. If he'd been the one completing the paperwork, Mulder might've been convinced his sloppy accounting caused this problem. Scully, making a mistake on an official report made Skinner's statement impossible.

'If you could both look over these, the areas of concern are highlighted,' handing several stapled papers across the desk, Skinner watched for any silent communication between the two. He wasn't disappointed.

'Sir,' Scully started, her gaze turning towards her partner for just an instant, in which an entire conversation proceeded. Returning her cerulean blue orbs back to her boss, a sigh escaped her cherry red lips. Special Agent Dana Scully decided discretion, in this case, would not be appropriate. She knew Assistant Director Walter Skinner knew. Only her partner remained clueless. The truth had finally been uncovered.

'This better be good, Agent Scully,' Skinner warned. He hadn't missed her small hesitation, or the defensive posture. The answer to all the questions he didn't want to ask lay in Scully's eyes, as always.

'Sir, if I may speak candidly,' Scully attempted to draw Mulder into the conversation, but he refused the invitation. A slightly sardonic smile covered his face as hazel orbs ogled the scene taking place before him. Forced to continue alone, Scully continued, 'these reports only act to confirm what you already either know or suspect.'

'You want to know what or who tipped off the Bureau?' Skinner couldn't hide his malicious smile.

'We have been very discreet,' Scully asserted calmly, more so than she let on. Beneath the surface, anger, hurt and disappointment roiled in the acknowledgment this day would come.

'Several days ago, one of Agent Mulder's sources stumbled into a hospital in Washington State, clutching his card. This woman refused to talk to anyone but Mulder. The Seattle field office contacted me with the case because they believed her injuries to be a direct result of a current investigation and requested the help of the agents assigned to the X-Files. They wanted the information this woman withheld. Friday night, neither you nor Agent Mulder could be reached at home or on your cells,' Skinner paused, his anger obvious.

'I believe we were at Mrs Scully's for dinner,' Mulder chimed in for the first time. Skinner rased an almost none existent eyebrow over the top of his round glasses. Feeling like a child caught with his hand in the cookie jar, Fox Mulder felt impelled to add, 'too much alcohol meant both Scully and I stayed the night. I forgot to charge my cell the night before,' Skinner's eyebrow rose further, if at all possible, but Mulder continued, currently placing both feet into his mouth. 'Scully had hers turned off.'

'I see you didn't want to be disturbed,' the sarcasm in the A.D.'s tone dripped over both agents, who took the opportunity to have yet another silent conversation in a single glance.

Taking her disbelieving gaze from her boss to spear her partner, Scully tactfully said, 'I don't understand what this has to do with our expense reports, Sir.'

'I sent a pair of agents to Mulder's apartment,' pausing to let that little kernel sink in, Skinner continued, 'considering the past situations you've fallen into, I felt discretion to be the better part of valour. Imagine my surprise when the occupant told us they'd sub-let nine months earlier. The reason Miss O'Donnell gave us, the original tenant couldn't break his two year rental agreement. Of course you can guess our next port of call, Agent Scully.'

'My home,' she answered quickly, one more glancing at her partner. This one easily read. It told anyone caring to look that they'd been caught. This time Mulder reached out a hand, laying it with long familiarity on Scully's arm in a gesture meant to comfort and calm.

'Given the circumstances, and given Agent Mulder's source in Seattle passed away under suspicious circumstances, and I'd discovered two of my agents missing,' allowing for a dramatic pause before continuing, Skinner continued, 'we had justifiable cause to enter.'

'I see,' Scully pursed lips communicated her sense of both disbelief and invasion of her privacy on what she considered a flimsy excuse at best.

'It's what we found that surprised us,' his chocolate orbs switched from one agent to the other, mentally flaying them. 'I don't believe we found a room in your apartment, Agent Scully, which didn't contain something belonging to Agent Mulder.'

'I don't think you're quite correct, sir,' Mulder chimed in ironically, his hands now neatly folded in his lap, 'Scully wouldn't let me bring anything from my kitchen when I move in.'

Giving him the stare of death, Scully attempted to get the conversation back to the expense reports by holding them aloft. 'I still don't see how this correlates with our domestic situation.'

'You've been married for nine months, and yet you're still charging the Bureau for two hotel rooms on field assignments,' Skinner hissed between clenched teeth. 'It's enough ammunition for some to close the X-files permanently. There are reasons why intimate relationships between colleagues are frowned upon. Between partners it's almost unviable as emotions get involved in the working partnership. Marriage requires both of you to be reassigned immediately. Is that what you want?'

'Up until Friday night,' Mulder reminded his superior, 'we maintained plausible deniability. Our work hasn't suffered. Our investigative resolution rate hasn't decreased, even if few of our cases make it into the legal system.'

'I can't sweep this under the carpet,' annoyed by the turn of events, Skinner continued, 'Apart from the team of four who initially swept Agent Scully's…I mean your apartment, six agents were assigned surveillance duty over the weekend because we couldn't contact you until Sunday evening. You'll be lucky if this isn't all over the building by the time you make it to your basement office.'

'I'm sorry we've put you in this position, Sir,' Scully apologised unable to look the A.D. in the eye. Mulder would never appreciate the consequences of crossing some lines, especially this one. For him, rules were made to be broken. He at least, didn't regret anything about their relationship, except keeping it from all but their closest family. That he'd done for Scully and her need to maintain her sense of professionalism and dignity. After all, they'd both heard the rumours about woman who slept with their partners.

'Too little to late, Agent Scully,' Skinner sighed warily. 'Tell me how it happened, how you managed to keep it secret and I'll see if there's anything we can do to mitigate the fallout.'

'It started with my cancer diagnosis,' Scully's expression turned serious as she remembered that awful time and how it became the catalyst for their current predicament.

'It started with you wanting your own desk,' Mulder quipped, still pissed by the assignment Scully investigated on her own. He loathed the fall out from that case even more when he learnt what his Scully had almost done. For the second time in as many investigations, she finished up in hospital. This time, however, Mulder's emotions, his anger had reached such a level he could no longer contain his frustrations. He crossed the line, personally and professionally with his harsh words towards Scully in their basement office. For him at least, that had been the catalyst.

'You have my complete and undivided attention, Agents,' Skinner attempted to remind the pair of his presence in the room. Looking deeply into each others eye, communicating on a level he envied, Walter realise this highly charged, overtly personal display demonstrate some things between them had never been settled. 'Obviously I don't need all the details, but enough to know how long this has been going on and how it started.'

'That started because of something Betts said to me at the end of our case in Pittsburgh,' Scully retorted with some heat, ignoring the presence of a third person in the room. They both hated their personal life being open to dissection and public interpretation. The fact that she would have to explain herself, lay open her action to anyone but Mulder, incited her anger. Opening up to her husband had proved difficult enough.

'What did he say to you in that ambulance,' Mulder asked, intrigued his wife hadn't told him. 'He knew you had cancer, didn't he. That's why he attacked you.'

'I had my first nose bleed the night we returned,' Scully confirmed.

'Dana,' his tone hurt, 'why didn't you tell me?'

'For the same reason you had my ova examined before telling me,' she issued.

Making a strangled noise, Skinner drew attention to his presence. 'I need to know what the hell is going on here,' he demanded.

Sighing, Dana Scully ignored Mulder's quiet pouting, as began the tale of her romance with Mulder.