Mags travelled to the planet Segonax with Captain Cook, an egotistical intergalactic explorer. Cook related that he had saved Mags when she was about to get shot with a silver bullet. Apparently, Cook wanted to somehow sell her to the Psychic Circus which had stopped on Segonax at the time.
The Seventh Doctor and Ace befriended her while looking into the Circus. Mags was later revealed to be a werewolf, her transformation triggered by moonlight. Mags seemed to revile and fear her transformations, and also feared the image of the moon itself. Cook managed to trigger her metamorphosis using artificial moonlight recreated by circus lights in an attempt to kill the Doctor for the amusement of the Gods of Ragnarok. The Doctor convinced her that she could control her feral instincts, but not before Mags ended up killing Cook himself.
After the Doctor had defeated the Gods of Ragnarok, she decided to stay behind on Segonax with Kingpin to re-start the Psychic Circus, despite her fears that her transformations might once again get out of control. (TV: The Greatest Show in the Galaxy)