This is a weird one - a Doctor Who / Diane Duane's "Young Wizards" crossover that works perfectly, and explains a LOT about Donna, the Tenth Doctor, etc. It helps considerably if you know both universes, of course. The crossover is (not quite explicit but almost) Young Wizards canon.
This is a link to the LAST part, which has links to all the rest
http://community.livejournal.com/myriadwords/56873.html