Episode 8.15

Of course, he ought to be devastated by the discovery, on Earth, of the 500-year-old remains of the severed head of his old friend and Number One: Tenbury Wells. But Alan has been doing his homework, watching rival old series finales of Star Trek The Next Generation. He is confident that, before the show is over, he will be presented, 500 years in the past, with the newly decapitated and bleeding body of his friend. 

No doubt, he can simply transport it back to their own time and reattach this convenient original skull. Job done.