War for the Nexus: a 6th Saviour story

The Saviour and the Sorcerer are alone in the TARDIS. They had just left Emily in the Saviour's version of Equestria, and were planning on visiting the Sorcerer's home universe. But, their pulled across Reality itself, and crash into a skyscraper.

Meanwhile, Mr L. Twisted was busy in his office, preparing to pack up and leave Imagination City for the night, only for a bright red Police Box to fly through his office, landing.

What happens now? What is the Sorcerer up to? And just what is the Nexus?