Time Out of Joint
A mysterious Immortal is jumping through time. He's harvesting heads and causing incalculable damage to the web of time. Trapped on their homeworld the Time Lords are powerless to intervene. In desperation, they disregard the first law of time and allow seven of The Doctor's incarnations to join forces to combat the threat and save all causality. If he can find a way.
Doctor Who/Highlander Xover.