 The series ' story arc was hinted at by the Doctor 's invocation of the Shadow Proclamation , an intergalactic code previously invoked in " Rose " , " The Christmas Invasion " , " Fear Her " and " Partners in Crime " , and referencing the " Medusa Cascade " , which executive producer Russell T Davies stated in Doctor Who Magazine would " come back to haunt us " later in the series . Moran also added continuity links independent of the story arc : as a " fun continuity thing " , the script includes a scene in which the Doctor admits partial responsibility for the Great Fire of Rome , as depicted at the end of the 1965 serial The Romans ; and the sale of the TARDIS as modern art refers to the 1979 serial City of Death , which includes a scene in which the TARDIS is appraised in a similar fashion . Additionally , Lucius Petrus , while under prophetic influence , informs Donna that " there is something on your back " — a foreshadowing of the story development in the season 's later episode " Turn Left " . 
