Warning: Spoiler ahead for Doctor Who season 15, episode 8, "The Reality War."Doctor Who introduced bi-generation in 2023's "The Giggle," with the surprise twist allowing David Tennant's Fourteenth Doctor to essentially split in two, spawning Ncuti Gatwa's Fifteenth while Fourteen remained completely autonomous. Fifteen's final episode has finally provided the best explanation yet for why it happened, and why Anita Dobson's Mrs. Flood also experienced the same phenomenon at the end of "The Interstellar Song Contest." Admittedly, it is only an in-universe theory, but it makes perfect sense for a moment of such throwaway speculation.