Katarina is introduced in the serial The Myth Makers, which takes place during the siege of Troy. She is a handmaiden of the prophetess Cassandra, and helps the TARDIS crew survive the events of the siege. When Steven is wounded, she helps him into the TARDIS as Vicki decides to stay behind in Troy with the warrior Troilus.
A sweet, simple young woman who cannot really cope with the concept that the universe has suddenly opened up to her, Katarina believes that she is dead, and that the Doctor is a god transporting her to the next life. She refers to the TARDIS as a "temple", and literally worships the Doctor, referring to him as "Lord" (much to his annoyance) and having absolute faith in him.
During The Daleks' Master Plan, Katarina is taken hostage by the escaped prisoner Kirksen, who demands that the Doctor take him to Kembel, a planet taken over by the Daleks. To prevent the Doctor giving in to Kirksen's demands, she chooses to trigger the controls to the airlock she is being held in, propelling both herself and her captor into the vacuum of space.