Momus the Wise was once a great computer. It was built by the Ridion Alliance's finest scientists. However, after the Seven Hundred Year War, his planet's population descended into a terrible dark age. All their great buildings fell into ruin, their sophisticated ideas were forgotten and their cultural achievements were lost. After a generation or two, they no longer understood what Momus was for. He was left all alone in the Palace of Hy-Ridion with nothing but his thoughts for company.
Momus intended not only to have company, but also fulfil his programming and help his people progress into a new age of enlightenment. Having mastered the K.R.H. Timeloop theory, he could travel through time and bring other objects and persons to him from other points in time and space. He therefore organised a "Great Dinner Party" for the greatest minds and talents of human history and the Tenth Doctor, accompanied by Donna Noble. The great Earth minds he assembled would help him the task of rebuilding his planet's civilisation.
The Doctor however made Momus understand that Earth needed the people he 'borrowed' and that his people needed to recreate their civilisation themselves. He convinced Momus to leave his planet and travel freely. Like the Doctor the last of his kind and alone, he was unsure how much he could leave behind the things he knew and loved, but promised he would do so. The guests were all returned to their proper places, and the Doctor and Donna were dropped off at Ardennes in 800 AD. (PROSE: The Lonely Computer)