The Sixth Doctor and Peri are in the TARDIS discussing "what-if's" to do with collapsing time-streams. They are interrupted by a voice who claims to have "created the creator", a being who has had a guiding and protective interest in the Doctor that goes back to before he left Gallifrey. The being claims to have made the Universe in a science lesson and placed a barrier around it to protect it from others of its kind and itself. Halfway through its existence it needed to have its Heart replaced. To this end he gave a Key into the charge of a Guardian of the Universe sealed inside.