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 characters listed below are the series regulars, important recurring characters, and real life historical characters. If a character's appearance is a major spoiler, then their details will be hidden in spoiler tags. In general, recurring characters are not listed here on their first appearance unless they become a series regular during the story, or they are a real life historical character.
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.
Recurring and Historical Characters
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.
The following stories and other resources are not necessary in order to follow the plot (we've summed up the essential bits for you). But being familiar with them will probably enhance your enjoyment of this story.
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.