In it, Spike bites Xander in such a way that makes him crave it, and crave it ~sexually. I remember he gets bit in front of Willow and enjoys it too, which upsets him. Then, Spike, magnificent bastard that he is, forces Angel to do the same thing to Xander too, which upsets a whole bunch of people. Except us readers. I remember they get stuck in a crypt while it's sunlight and Buffy finds out then that Angel and Xander are reluctantly getting it on and gets very sad/angry.
Any help would be greatly appreciated!