El último de los Señores del Tiempo
The episode features the return of the Sontarans, who last appeared in the 1985 serial "The Two Doctors"; a central appearance by UNIT; and Martha Jones, who had last appeared in "Last of the Time Lords" and made special guest appearances in the "Torchwood" episodes "Reset", "Dead Man Walking", and "A Day in the Death." The brief that executive producer Russell T Davies gave to writer Helen Raynor included the terms "Sontarans", "military", and "Martha's back".