The mission was dual purposed. On the surface, One and Two were sent to the wider universe to find out what had caused a Celesti to not only cease to exist, but to have never existed. During that investigation, though, Two was to watch One, as the Celestis believed it may have gone rogue. If this was confirmed, Two was authorised to eliminate One.
During the course of their mission, One found out Two's true purpose. It attacked Two and bashed its skull into a pulp. One left, thinking it had killed Two. However, the nanites in Two repaired the damage. There was still massive memory loss, so Two believed its mission was to kill any humanoid.
Eventually it regained enough memory to remember Mictlan and TARDISes. It wanted to go home. When it found Hume, it decided to eat his brain and absorb his knowledge so it could build at least a rudimentary TARDIS and return to Mictlan. While Hume and Two fought, Fitz Kreiner was able to finish repairing the Celesti fictional generator. He used it to turn Two into a work of fiction, the 1930s looking magazine Spicy Archon Stories. (PROSE: The Taking of Planet 5)