Interesting idea for a backstory to help explain his choice of taking the unbreakable vow. 

With Bella provoking him, Snape could barely have avoided the UV without loosing his position, I thought. But your story gives us the background why Narcissa visits him and confides in him at all. I've always wondered how she even knew the dreadful place he calls home. Now I understand that she has investigated into this direction for months. Besides, it is a great story all in itself. Rich Lucius keeping the beautiful puppet to show off while amusing himself with younger and ever younger girls. Severus, the tragic person, always falling for taken women and ending up protecting their spoiled brats. 

wonderful story Ladies. i do think that JKR knew a lil more about theses two than she let on.

Stupid Lucius, indeed.. and if only Severus could get over, ah, Mrs James Potter. Ever since reading Spinner's End I've liked the idea of a possible S/N relationship. I love this beautiful backstory. But however much I like it that he took the Unbreakable Vow for Narcissa as well, I believe that for post-Lily-Voldemort-fiasco-double-agent Severus she was not the only reason.Any hope for another S/N story?

Quite a strong story.  Worthy of an extended development with a slowly growing relationship that is independent of attempting to explain canon.  

Oh, yes, this is a very plausible reason why Severus would grant her the vow without question. It was hard to see him fail to reach release, I assume because of Lily, but that he cared enough for Narcissa to reach hers. I half expected Lucius to find out... and when you said five months later with tears in her eyes - I thought 'pregnant?' LOL. This was beautifully written.

Hmmm. I can certainly see such a brief affair happening. I can certainly see it being one of many reasons why he took the Vow. But I'm not quite sold on it being a front-runner as THE reason why he did it. The presence of Bat-shit Crazy Bella, amongst other things, still pops up in my mind as a driving cause.I'd be interested to see how he longstanding (though unfulfilled until now) relationship with Narcissa fit in with his feelings towards Lily.

I buy that he would do it for Narcissa, but I also would believe that the Slytherin part in her used that night and his feelings to bring about this end. Not that she knew she would need it so soon, but she wanted a bargaining chip later. That's just what I think.

I agree completely. Though I always imagined it was a friendship with thoughts that were never realized. I think this version makes a lot of sense. Being married to an iceberg would she not crave passion and attentiveness? Severus has both in spades!! You've really captured a plot-thread that could be part of canon, left untold.