Marni was twice married and divorced as a result of her dedication to her career. She was the daughter of the chief of police of the western militia. She wanted to be a law office from an early age and joined the force at the age of twenty-two.
She was assigned to the case of twelve murders. Marni survived the pressure waves released by the CVE. She was summoned by the cabinet and questioned on her case. There was an apparent connection between the murders and the CVE. Marni was ordered to travel to Letrus to investigate a further twelve murders there.
While on Letrus, she questioned Romana II and Adric as suspects. She deduced that they were not the murderers but asked to see their ship. The Farrian showered the area with teleport disk, teleporting Farrian near Marni and so she ran away.
After using her communicator, the Farrian traced her signal and bombarded her. The Doctor rescued her in his TARDIS and took her through a Farrian-created CVE into N-Space. It transpired the Farrian were responsible for the murders. The Doctor took her to the Farrian building generating the CVE. He set it to overload, destroying it. He returned her after the Farrian were forced to retreat. (AUDIO: The Invasion of E-Space)