Doctor Who: The Newbies Guide
This is the sixth story in Season 3.
Where does this fit?
This happens very shortly after The Massacre
The first Doctor looks like a human in his sixties. He is physically quite frail, but has a keen intellect. He is paternalistic towards his companions, and has a degree of arrogance towards others. He sometimes mispronounces words.
Steven Taylor was a space pilot from an unknown time in the future. His spacecraft landed on the planet Mechanus, where a group of robots called Mechanoids held him prisoner. In The Chase, he escaped when the Doctor, Ian, Barbara, and Vicki visited the planet whilst on the run from a Dalek time machine. He stowed away on the TARDIS not knowing it was capable of time travel.
Dorothea Chaplet (commonly referred to as Dodo is a girl from 1966 England who (in The Massacre) ran into the TARDIS on Wimbledon Common, thinking it was a genuine police box.
Recurring and Historical Characters
Dodo was introduced in the final scene of the previous story (The Massacre).
If you would like a proper origin story for Dodo before she joined the TARDIS, that can be found in the BBC Past Doctor novel Salvation.