The Mad Hatter was a character who existed in the Land of Fiction. When the Seventh Doctor was wandering in the void outside Arandale, he encountered the Hatter, who invited him to join his tea party; the Doctor declined and headed on towards Arandale Keep. (PROSE: Conundrum)
Behind the scenes[]
The Mad Hatter and his tea party come from Alice's Adventures in Wonderland by Lewis Carroll.