It was as this incarnation that the Doctor first met the Daleks and the Cybermen, races that would become his most implacable foes. The TARDIS crew also observed many historical events such as the "Reign of Terror" in revolutionary France, meeting "Marco Polo" in China and "The Aztecs" in Mexico. When Susan fell in love with the human resistance fighter David Campbell, the Doctor left her behind to allow her to build a life for herself on 22nd century Earth ("The Dalek Invasion of Earth"), although he promised to return some day. The Doctor, Ian and Barbara were then joined by Vicki, whom they saved in "The Rescue" from the planet Dido.