I'm gonna try my best to write this ON THE SPOT without spoiling anything too much. Beware of spoilers for the TV series.
OAV Part 1:
Basically Yui steals the book which alters the destinies of everyone. Tamahome and Nakago switch souls. Yui becomes the new Gembu No Miko which erases the original Miko, Takiko. Apparently another screw-up was the double-miko summoning, which threw the 100-year cycle off. The 100 year cycle is, obviously, a miko will appear every 100 years. The new miko will open the book and erase the previous miko. This goes on forever, apparently. o.O;
What will happen to Yui, Tamahome, and Nakago? Get the tapes for yourself, or read my synopsis. :P
OAV Part 2:
Tamahome's been reincarnated as Sukunami Taka, who has no previous memories of being a Shichiseishi but knows SOMEHOW he knew Miaka in a previous life. Taka must find the 7 memory stones to, of course, regain his memory. Getting Tasuki and Chichiri's was simple, but of course, to every up there's a down. Taka had to deal with a whole bunch of screw-ups. (like the memory stone inside Hotohori's son, Nuriko's brother being an idiot..^^;) Will Taka be able to get the memory stones before the big bad Tenkou gets them? Watch and see.