One function of the episode is to reveal that the Doctor is the last of his people. Davies had also initially intended to have the last humans escaping the doomed Earth aboard massive space arks (similar arks appeared in the serial "The Ark"). This was dropped when the complexity of the character of Cassandra was fully realised during development. The episode features the first appearance of concepts such as the psychic paper, the Time War and the words 'Bad Wolf', which would go on to form a story arc throughout the series. It also mentions the TARDIS' universal translation capabilities, which had been referenced in the 1976 serial "The Masque of Mandragora". An unaired scene would have shown the Doctor to have nine strands of DNA instead of one, an allusion to his eight previous incarnations.