F or production purposes, each individual Doctor Who serial was given a letter code – but even though the one-off episode Mission to the Unknown followed Serial T, Galaxy 4, it wasn’t known as ‘U’. Instead, it was Serial T/A – a bonus episode to make up for the fact that the previous year’s Planet of Giants had been cut from four episodes to three in post-production. Which was how this prologue to The Daleks’ Master Plan, the only episode of Doctor Who to feature neither the Doctor nor his companions, came about.