1957 New Mexico, the Doctor, Benny and Ace encounter the Master, who has used nanotechnology provided by a race known as the Tzun to restore and regenerate his body. Later, the three travelers meet one of the oldest beings in the universe, a "grey man" who tries to weaken the good-and-evil dualism which his people had instilled in the universe's structure. On Earth in the 21st century, the Doctor, Benny and Ace investigate a new drug called "warlock" which has the power to enable the user to transfer his or her mind to another place or body; they discover that drug is actually an alien gestalt intelligence, and help it to leave the Earth. A series of rifts in time and space (created by a crude time machine used by Kadiatu Lethbridge-Stewart) sends Ace to Ancient Egypt, Benny to France in 1798, and the Doctor to the Paris Commune of 1871; the three are eventually reunited, but Ace decides to stay behind in Paris, keeping one of Kadiatu's time hoppers. She takes the title of Time's Vigilante. After Ace's departure, Benny has a series of painful losses. First, her friend Darius Cheynor (a 24th century officer first encountered in The Dimension Riders) survives an encounter with powerful Sensopaths from the end of time, only to be killed in a conflict with the cybernetic Phractons. Shortly thereafter, Bernice falls in love with the 13th century Knight Templar Guy de Carnac, who apparently dies defending a Cathar village in the Albigensian Crusade. Unable to understand Benny's grief on a human level, the Doctor purchases a device which alters his biodata, transforming him into a human named Dr. John Smith. Smith lives as a history teacher at an English public school, and falls in love with a fellow teacher named Joan. However, when alien Aubertides, hoping to acquire Time Lord abilities, attack the school, Smith sacrifices himself and becomes the Doctor once more; as the Time Lord, he is unable to love Joan in the way the human John Smith did. Joan gives the Doctor her cat Wolsey. Investigating a mysterious warning about Earth in the 30th century, the Doctor and Benny meet Adjudicators Roz Forrester and Chris Cwej. The four discover extensive corruption in the Earth Empire, and a trail which leads them to the Doctor's old enemy Tobias Vaughn, who had survived his betrayal by the Cybermen and worked for centuries