This one had Spike running into Hyena!Xander and taking him back to a hotel. There may or may not have been hothotsexxins before or after the hyena was exorcised. There was sex, I'm just not clear on when it happened. But it happened!
And then Spike went and got the whammy put on him with an option for Xander, so that he'd forget it ever happened. Something about Dru knowing that Spike was going to betray her and him taking the spell/potion to protect Xander from being a target if Dru found out -- like if Spike didn't remember, then Xander was safe.
Anyway, Xander had the option to forget and never took it, so when Spike showed up again later, and then was staying in the basement o' doom with him (way later), HE still remembered that three-night stand, but Spike didn't. Drama ensued.
I do believe there was a happily-ever-after in there somewhere.