Aldridge was a Xing surgeon located in the Strand London in 1900. He was a professional surgeon, however, his location on Earth was frowned upon and so he kept a signal around himself in order to hide from the rest of the Universe.
In 1900, the First Doctor visited him in order to replace a hand. Aldridge willingly obliged, but asked for an explanation. The Doctor told him that he had lost it during a battle with Soul Pirates. Aldridge assured the Doctor that a new hand would be ready within five days, but the Doctor demanded two.
He later returned with his granddaughter, Susan Foreman, who obligingly told Aldridge about their adventure while the Doctor slept after the operation. Aldridge was intrigued, and realised what an open-minded Time Lord the Doctor really was. (A Big Hand for the Doctor)