The Doctor To the Rescue
There's never been any real, official clarification how long the Doctor may have travelled with Susan. Being his grand-daughter, its possible they travelled together for many years before settling on Earth. The Doctor locked Susan out of the Tardis in hopes of helping her 'grow up'. It may have seemed like a cold, tactless thing to do, but he truly missed her once she was gone. This was clearly seen after the Tardis lands and he absent-mindedly instructed Susan to open the doors. He paused in reflection for a moment when Barbara suggested he show her how to operate the doors.
With Carole Ann Ford having left, a replacement was needed. The series was to continue for another season so Maureen O'Brien (Vicki) was selected as the replacement. On the planet Dido, the year is 2493 and Vicki is one of two survivors of a crashed rocket from Earth. They are both terrorized by a mysterious being known as Koquillion who claims to be the reason they are still alive, and deserves their respect.
Relations between Vicki and the Tardis crew are strained when Barbara kills Vicki's pet 'Sandy', one of the native Sand Beasts. Despite starting the series as a cranky, self-absorbed personage the Doctor shows his softer side by reassuring Vicki they are there to help. It seems the departure of Susan has mellowed the Doctor, as well as his continued travels with Barbara and Susan.
Turns out the Doctor is rescuing Vicki from more than just a crashed rocket. Koquillion is actually Bennett (the other survivor) in disguise and murdered all crew members from the crash. Bennett however, underestimates the Doctor and is killed by his own stupidity. Vicki is brought aboard the Tardis and she marvels at how 'it's bigger on the inside'.
PLANET OF ORIGIN: Earth, circa 2493 - though found on Dido.
REASON FOR JOINING: Rescued from a crash landing and murderer.
Next: The Daleks Give Chase