Centuries before the Doctor's time, Tamira had been travelling in the Time Vortex in her TARDIS when the Thrusk ambushed and infected her. After failing to eject them by regenerating, the Thrusk froze her body in stasis and secreted her inside her own TARDIS, inside a dimension within a Time-Space Visualiser.
Many years later, Mel investigated the world within the visualiser and together with the Sixth Doctor, they met Tamira and the Doctor managed to free her from the Thrusk. Tamira told the Doctor and Mel in the Doctor's TARDIS she would return to the Gallifrey of her time and she promised to keep the secrets of the Type 40 TARDIS so the Time Lords of her time didn't advance too quickly. She then departed in her TARDIS. (AUDIO: Mel-evolent)