Jo Martin was introduced as The Doctor in "Fugitive of the Judoon". Martin's main character in the episode is Ruth Clayton, a tourist guide who lives in Gloucester with her husband, Lee, who is perceived to be the fugitive that the Judoon are looking for. Later in the episode the Doctor and Ruth visit Ruth's childhood home, an uninhabited lighthouse where Ruth believes her parents are buried. The Doctor traces a signal back to the gravestone where she digs out a TARDIS. Martin's Doctor then appears and introduces herself as the Doctor in full costume. Neither Doctor recognizes the other. Towards the end of the episode, both Doctors meet Gat, an unknown Gallifreyan woman working with the Judoon to find the fugitive, who was in fact the Doctor. She is last seen asking Whittaker's incarnation to leave her ship.
Jo Martin -- Who Not Who!