John Henry was a shabby green Door who participated in the Door invasion of Earth in the Edwardian era. John Henry didn't originally have a name; the Tenth Doctor gave him "John Henry" in tribute to Joseph Henry.
Before coming to Earth, John Henry mentally connected with an advanced artificial linguistic intelligence, giving him the ability to speak in all human languages. He was chosen to act as a translator between his people and humanity.
Shortly afterwards, John Henry met the Tenth Doctor and was given his name. The Doctor gave John Henry a doorbell.
During the diplomatic meeting between the Doors and the government, John Henry realised that his people didn't want to peacefully coexist with humanity. At John Henry's consent, the Doctor connected the sonic screwdriver to John Henry's doorbell and used it to create a vibration which rang throughout the dimensions. The vibration paralysed all the Doors, making them outwardly no different from regular doors. John Henry was installed as the front door of 256 Beaton Square. (PROSE: Knock Knock!)