Doctor Who fans are losing it on social media as a beloved character long since seen on screen made their grand return in the show's latest episode. Season 13, a multi-part epic subtitled Flux, reached its penultimate installment this Sunday. Called "Chapter Five: Survivors of the Flux", it introduced another thread to the interconnected, complex plot that reintroduced alien-busting organization U.N.I.T. - along with its head, Kate Stewart (Jemma Redgrave).