Captain Summerfield was a member of Brigadier Alistair Gordon Lethbridge-Stewart's special forces when he first moved in to the new HQ, serving as the Brigadier's aide and second.
In the original track of time, he served the Brigadier faithfully for the following fifteen years. However, Deller, a scientist with a grudge against the Brigadier, sent a robot duplicate of Summerfield back in time to this first day at the new HQ and had him murder and replace Summerfield. The duplicate was discovered and Deller's wider plan foiled, but this meant that in the new form of history, Captain Summerfield could no longer assist the Brigadier throughout his life. (AUDIO: Memories of Tomorrow)