The Massacre, also known as The Massacre of St. Bartholomew's Eve, is an episode of Classic Who with a somewhat troubling title. It stars Billy "the Kid" Hartnell, but not as who you might think, and debuts the most ignored of all classic companions. It was written by BRAVISSOMO LUCAROTTI. It's a pretty dire historical, even by dire historical standards. Unfortunately(?), we'll never know for sure, because it's gone.
One of the few parts the story is clearly remembered for is its ending, which is pretty heartbreaking because, for the first time, the Doctor is left alone. Properly alone. Totally abandoned by his only companion, sitting alone in his TARDIS and wondering if he should return to Gallifrey before declaring he can't.