This is a rarity for Doctor Who twofold over. First, it's a multi-Doctor adventure, which had only been done once before and has only been done twice since then... as of this writing, anyhow. Second, this was also the 20th anniversary special for Doctor Who - aired during Children in Need night (though US broadcasting was two days before, on the actual anniversary date of November 23). Finally, this was presented as a Who-movie in one massive part rather than a multi-part adventure (though it didn't stop the BBC from turning it into a 4-parter a few years down the road). The Master, of course!