THE COLONY IN SPACE 1.

The Time Lords permit the TARDIS to make a voyage through Space and Time for more than a year.
The Master has stolen the Doomsday Machine file from the Matrix, and it must be retrieved.
The Time Lords despatch the Doctor and Jo Grant to a bleak Earth-type planet in the year 2471, where
they meet the Colonists, people who left Earth because of overcrowding.