There are several references to the paradoxes of time travel. Martha mentions the possibility of killing her grandfather, an allusion to the grandfather paradox, when she first steps from the TARDIS. She also suggests that stepping on a butterfly might change the future of the human race, an idea that originates in Ray Bradbury's 1952 short story "A Sound of Thunder". The Doctor explains how history could be changed with devastating results by referring to the movie "Back to the Future". Martha scorns this explanation by saying 'The film?' to which the Doctor retorts 'No, the novelisation! Yes the film!'. There is indeed a novelisation of "Back to the Future", written by George Gipe.