Having reached the end of her life and approaching death, Galah linked her mind to her TARDIS's Protyon Unit at the moment of regeneration. All the energy poured into the TARDIS, and utilising the Architectural Configuration Programme with a link to the real Victorian Earth, Galah created an artificial universe within the Matrix and even created living beings, albeit TARDIS constructs. The TARDIS became a "good" universe, modelled on a Victorian English house in summer, with twelve people who lived there and maintained the house, not knowing anything like cold or death or ageing.
Accidentally arriving within the program, the Seventh Doctor, Ace, and Benny's presence upset the universe and caused "Elementals" to begin killing people, lead by the Quack who embodied the TARDIS trouble-shooting program.
Ace was expelled from the universe to Earth, but returned via the link with Dr Patrick Rix. Rather than be killed by the Elementals, Rix committed suicide, but everyone who died in the artificial universe was assimilated, leading Rix to take control from Galah and reshape the English summer into a hellish plane. Combining their wills to trick Rix with his memory, the Doctor and Galah erased him from the Protyon Core.
Because Charlotte Aickland was the only construct to survive and was "alive," the Doctor persuaded Galah to pour some of her energy into Charlotte to make her real before dying. The TARDIS was left behind, the good universe dying with it. (PROSE: Strange England)