It's 1913 in England, and an ordinary schoolteacher called
is disturbed by dreams of adventures in time and space and a mysterious
blue box. But, when lights in the sky herald the arrival of something
strange and terrible, Smith's maid, , has to convince him that he alone can save the world.John Smith's journal features sketches of the interior of the , a , , , , , , , the and as well as a picture of the gas-mask plague carriers from ""/"". Also the , , , , and Doctors appear, the first time in the new series that the previous incarnations have been explicitly acknowledged on-screen..As part of his cover story, John Smith refers to his parents and , a reference to one of the creators and the first producer of Doctor Who respectively. Russell T. Davies confirmed this in .Fleeting clips of the lone Dalek in "", Cybermen, , , the seen in "", and in his mutated form are shown when Timothy opens the watch. There is also a clip of the Doctor using his in "" and "".John Smith's skill with the cricket ball is reminiscent of that of the , best exemplified in .In the flashback the Doctor mentions a Time Agent vortex manipulator.The melody used while Lucy is walking down the path is similar to the melody used in when the Girl appears.The Family's possession of Lucy Cartwright is a nod to the
character of Aphasia in the original novel, a shapeshifting Aubertide
who likewise takes the form of a young girl with a balloon.The Doctor explains he will cloak the pocket watch with a "perception filter". The episode "" coined the phrase when stating that some of the 's properties became welded to due to the activities of the . In Torchwood, the perception filter cloaks the Torchwood Hub's lift from the untrained eye.