Though short, this is by no means a simple challenge.

S2. Either Buffy or Drusilla is cursed, but both are affected by it. By day, Buffy is herself, but by night she and Dru switch bodies. Up to you if this happens before School Hard, and is perhaps the reason Spike and Dru come to Sunnydale, or if it is after.

Bonus points for Buffy having to deal with being Dru while she is weak and sick.

Have fun :D

Puppet.