"One" is the thirty-first series finale of the British medical drama television series Casualty and the 1,049th episode of the overall series. The episode is written by Paul Unwin, the show's co-creator, and directed by Jon Sen. It premiered on BBC One and BBC One HD on 29 July 2017. "One" is a fly on the wall episode following an hour in Holby City Hospital's emergency department (ED) with additional focus on sister Lisa "Duffy" Duffin (Cathy Shipton) mentoring two work experience girls and paramedic Jez Andrews (Lloyd Everitt) grieving the death of a baby.