It's not really a crossover, I haven't seen enough CSI for that. The basic idea is that someone at FBI notices this little town in California with a totally insane unexplained death rate, and further suspects that the local police may be involved. So they send in an outside group (the PCs) to try to figure out what the heck is going on.

To be honest, it'll probably be more X-Files than CSI. But it was the "CSI: Sunnydale" title that gave birth to the idea.