"part one" features a flashback of david tennant as the tenth doctor using previously broadcast clips from "school reunion" and "journey's end". sarah jane mentions that she knows "what it's like to be taken over". previously, she had been possessed in "planet of the spiders", "the masque of mandragora" and "the hand of fear". the idea of godlike entities from the universe before our own, deriving their powers from alien laws of physics which no longer apply, is a nod to virgin publishing's "doctor who new adventures" range, for which scriptwriter gareth roberts wrote three novels. the draconians are mentioned in "part one". they featured in the "doctor who" story "frontier in space". clyde mentions luke nearly bringing the moon crashing down on earth in "the lost boy". sarah jane speculates that martin uses a biodamper, a device first seen in "the runaway bride".