Milledge was a member of the United States Secret Service. By 1969, he and Spencer had known of the species that was sending a transmission originating from the Moon offering to aid the development of humanity. According to the Tenth Doctor, the message was hidden from the general public, saying the "pathetic" agents "locked this transmission away in a box for their own ends", to "try and pervert" the original message into a weapon to win the Cold War.
As the Apollo 11 mission would approach the Moon, where the transmission originated, on the day of launch, 16 July 1969, Spencer and Milledge took command of Mission Control. After the Doctor threatened to stop the agents, he ran towards the main speakers with a dictaphone. Milledge warned no-one to help the Doctor, calling him a communist spy. One of Mission Control, Justin, helped the Doctor anyway by turning the speakers up. After recording the transmission, the Doctor surrendered to the agents, and Justin was imprisoned with him for aiding and abetting.
On 20 July, when Neil Armstrong landed on the Moon, Milledge noticed that no-one else was present. Milledge started "You think the Doctor—?" before Spencer shot down the insinuation that he could have gone up to the Moon and "asked the saucers politely not to contact us after all". With nothing to show, the agents quietly left. (PROSE: Blue Moon)