So I've had this basic idea for a story that's branched off into two different directions. One I've thought about a little more, and the other...not so much. I'm willing to try either one, though.
The basic premise is a woman tells a close male friend of hers that she's been having the most unusual dreams: every night, she dreams she's having the best sex of her life with an unknown partner. Little does she know that she's not only not dreaming at all, but the friend to whom she's confessing is responsible. He takes pleasure in this new knowledge and continues to visit her at night, while trying to help her cope during the day, until one night something happens (or rather, doesn't happen) and she finds out exactly what's going on...
The first direction I thought of is the one I haven't tried to develop very far. The friend is an incubus (or some other type of supernatural being) and it's in his powers to turn sexual encounters into nothing more than a dream. The reason I haven't developed this as far is because I can't think of what might go wrong so she's unaffected one night. If you're up for this direction, we can put our heads together and figure something out.
The second direction is a little more mundane. The friend is human, but he's a drug chemist (or has a buddy who is.) He (or his buddy) develops a drug. Maybe it's a sleep-aid that wasn't quite made right, and it makes the person aroused but affects memory, turning the encounter into a "dream." Maybe the drug was developed specifically as a new date rape drug, making the victim extremely responsive to sex but wiping her memory (the dreams may or may not be a desired side effect.) Anyway, the story would begin the first night he slips her the drug. Why he does so is up for debate. Maybe he asked her out, and she shot him down. Maybe she's got a boyfriend who's out of town/overseas/somewhere else far away so he doesn't get involved, but she's not interested in a sexual relationship with someone else. Maybe kinky sex has come out in conversation, and she insisted she's very vanilla, so he decides to prove to her that kink is fun. Or maybe you can think of another reason. I'm thinking he slips it into a drink she has every night (like wine or hot chocolate) so he can continue to dose her in the future. Maybe he means to try it just once, but gets a kick out of the encounter, or maybe he goes into this with every intention of dosing her every night. I'm thinking he's maybe a room mate, which would not only give him easy access to her food and drink, but also to her bedroom. Barring that, he's a close friend and has a key to her place, and he finds a way to dose her supply of whatever item he's using. Or they could happen to work together, and they're pulling late hours working on an important project. Anyway, one night she doesn't consume enough of the drug for it to take effect (or maybe the drug has a short shelf life and expires, making it inert) and he doesn't realize it, so she's lucid when he comes into her room. What happens from there is up for discussion: he backs off and fesses up, she's so aroused by all the things he's done to her in her "dreams" that she decides to become his pet, he rapes her...
While there would be a lot of sex in this plot, no matter which direction we take it, I'm hoping to balance it out with playing around with their inner thoughts about this whole thing: how she's waking up in the morning exhausted and sore, as if she'd spent the night having wild sex, and she can't understand why...maybe it's making her paranoid (or the drug could be having a lingering effect and making her more promiscuous through the day...not to the point where she's spreading her legs for every male she sees, but making her more flirtatious and teasing); how he's enjoying watching her change, and/or getting a kick out of playing the concerned friend when she vents to him about her strange dreams.
I think that's about it. I know that's a lot...but I've been thinking a lot about this story over the last couple weeks.
If you're interested, either PM me or post here. Edit: Thank you, I've had a lot of interest in this idea, so I'm locking the thread to see how these ideas play out.