The Doctor and Steven are amazed to find Dorothea "Dodo" Chaplet stumbling into the TARDIS in 1966 London, thinking it was a poilice box. The pair have just left a strikingly similar woman to her fate in The Massacre in 1572 France, and now are confronted with her likeness. We soon learn that the young teenager must be a descendant of the French woman and the Doctor allows her to join their adventure.
Dodo's personality was an unsophisticated, bright and happy one. In many ways she reminded the Doctor of his grandaughter, Susan, and was perhaps why he allowed her to remain onboard the TARDIS
After traveling to the far future and spreading the common cold in The Ark, and being a witness to the shoot out at the OK Corral in The Gunfighters, Dodo stays on with the Doctor when Steven decides to stay behind in The Savages.
Unfortunately, Dodo then falls victim to the mind control of the evil computer Wotan in The War Machines. Her instructions are to kill the Doctor. Thankfully the Doctor is able to use his own considerable mental abilities to break her free from Wotan's control, but Dodo is severely weakened by the ordeal.
The Doctor sends her off for a long rest in the country, a long rest that apparently turns into forever, because we never see her again. We learn later secondhand from Polly, that she has decided to remain on Earth