Ruth Matheson was a UNIT commissioned officer, with the rank of Captain, who for a time, was placed in charge of the Vault, an archive where many of the alien artefacts found on Earth were housed during the 20th and 21st centuries.
She was part of a United Nations peacekeeping unit in Bosnia after the fall of Yugoslavia in 1993. When she was there she helped to evacuate a group of children from a battle zone, when her unit was killed in a bomb blast.
In 2011, Matheson gave Charlie Sato a tour of the Vault when he first became its warrant officer. In the Vault, they came across four objects: An army jacket, a painting, a crystal and a wax cylinder, which bore a significance. Each was used in one of the Doctor's many adventures. (AUDIO: Tales from the Vault)
Matheson and Sato were in the vault when the Master awoke and went to visit him. She interviewed him as part of the protocol that the Brigadier set out and helped Sato understand why he was in the Vault. The Master tried to manipulate her by telling her that if she set him free then he would stop her unit being killed. The Master managed to succeed in tricking her and Sato to allow him to escape in his TARDIS. (AUDIO: Mastermind)
After the escape of the Master, Matheson, along with Sato, was relieved of her duties at the Vault and "reassigned", having been deemed a Category-One Security Risk due to the risk of the Master placing any post-hypnotic suggestions inside her mind. Relocated to a private residence for compromised UNIT personnel, Ruth believed that any tampering made by the Master would wear off in time, and so kept herself busy by completing several degrees, unlike Sato who felt that his career, and life, were now over. However, they earned a chance at a reprieve when they were recruited back into action by Mike Yates, who needed their help to investigate what had happened to the Vault during a communications blackout and subsequent loss of contact with several investigative teams. Although it was mostly expected by the higher-ups that the assignment would be a “suicide mission”, both Matheson and Sato were able to survive, and Yates promised to make sure that their security status would be reevaluated and they would return to active duty. (AUDIO: The Screaming Skull)