Dodo is introduced at the end of the serial The Massacre of St Bartholomew's Eve. In that story, the Doctor and Steven travel to 1572 Paris, where they witness the persecution of the city's Huguenot population. Despite befriending a young woman named Anne Chaplet, the Doctor knows he cannot prevent the coming massacre of 10,000 Huguenots, including Anne, by the Catholic French authorities. He therefore leaves in the TARDIS, taking Steven with him. When Steven finds out, he is furious and considers leaving the Doctor while the TARDIS is in 1960s London.
Steven returns at the same time that a young woman wanders into the TARDIS thinking it was a real police box. The Doctor and Steven are taken aback when she introduces herself as Dodo Chaplet and reveals that her grandfather was French. The Doctor speculates that Dodo might be Anne's descendant. As some have pointed out, this would mean Anne lived to have an illegitimate offspring.
In her travels with the Doctor, Dodo travels to the far future, unfortunately bringing the common cold with her to infect humanity's descendants; faces the mad games of the Celestial Toymaker; witnesses the gunfight at the O.K. Corral; says good-bye to Steven in The Savages; and is hypnotised by the rogue artificial intelligence WOTAN in The War Machines.
Halfway through that last adventure, she abruptly departs for a rest in the country after being hypnotised, and never reappears. At the story's conclusion Polly explains to the departing Doctor that Dodo has decided to remain in the 20th Century.