Mary left Earth with her father in 2471 to build a new life on another world, but the colony experienced nothing but problems. When the colony's problems came to a head and people were talking about leaving, Mary elected to stay like her father and give it another try. When the Third Doctor and Jo arrived, Jo went with Mrs Leeson to the mess hall and meet Mary and talked with her about Earth, and also broke the shocking news to Jo about when and where she was.
Operating the radio set, Mary was the first to receive news from the Leeson dome that they were under attack by some kind of giant lizard. Then a mysterious man was found wandering near the colony's domes; she and David Winton helped him recover. She and Jo became friends and worked together preparing food and setting up things.
Later, after Winton made up his mind to attack the IMC spaceship, Mary volunteered to help distract the guards so that the Doctor and Winton could enter the ship. When the adjudicator (i.e. the Master) arrived, she helped her father greet him and set aside space for the inquiry. When Winton refused to accept the adjudicator's decision, another fight broke out in which the colonists defeated IMC and she helped with the wounded.
Later, when IMC came back and finally beat the colonists and sentenced them to exile, Mary was put in charge of the personal checklist and oversaw that everyone was on the colonists' spaceship. She confronted Caldwell about their chances of surviving a lift-off in their antiquated spaceship and tried to convince him to help them. After the situation was resolved with IMC and the Master and the problems with their crops overcome, Mary welcomed Caldwell to their community and went about her duties to help her people build a colony. (TV: Colony in Space)