The Duke of Manhattan was a title given to one of the leading figures in the city of New New York on the planet New Earth. He had the power to create legally binding contracts on behalf of the Senate.
In the year 5,000,000,023, the Tenth Doctor and Rose Tyler encountered the Duke of Manhattan in a hospital run by the Sisters of Plenitude. He was suffering from Petrifold Regression, a disease that turned its victims to stone. He claimed that he suffered this disease despite having lived a life of charity and abstinence. Due to advances in medicine made by the Sisters, they were able to cure his disease, a cure that would subsequently be lost for another thousand years following the collapse of the order. The Duke was accompanied throughout his stay in hospital by Frau Clovis. He came to see the Doctor as his good luck charm. Though obviously regarded as much more important than most of the other people present in the hospital, he was willing to help in dangerous situations rather than rely on others to protect him as he personally helped his fellow survivors barricade the doors against a horde of plague-infected new humans. (TV: New Earth)
The Duke of Manhattan was recalled by the Doctor when he returned to New New York 30 years later, asking if there was a way of getting through to him after an attempt to call the police failed, only to be told he couldn't make outside calls as he was on the Motorway which was completely enclosed. (TV: Gridlock)
The senate passed several decrees regarding the Duke including: