The couple seen in the pre-credit sequence are made up to resemble the father and daughter in the Grant Wood painting, "American Gothic".
As The Doctor and Martha exit the TARDIS onto New Earth, The Doctor pulls an arrow out of the TARDIS door, an arrow which Queen Elizabeth's men shot at them at the end of the previous episode, The Shakespeare Code (2007).
This episode formed the final chapter in a loose trilogy about the fate of the human race after the expansion of the sun destroys the Earth in the year 5,000,000,000. The previous two episodes in this trilogy were The End of the World (2005) and New Earth (2006).
The Macra first appeared in The Macra Terror: Episode 1 (1967), and the Doctor's description of Gallifrey is identical to that given by Susan in A Desperate Venture (1964) within "The Sensorites" serial.