Corporal Forbes was a UNIT soldier who served under Brigadier Lethbridge-Stewart.
Forbes, along with another UNIT soldier, was posted by Captain Munro to guard the Doctor's TARDIS and was issued live ammunition to do so. Forbes challenged Sam Seeley when the poacher stumbled across them. He was later posted by Doctor Beavis at Ashbridge Cottage Hospital to guard his car, but was then picked up by his UNIT colleagues when they discovered one of the Nestene meteorites. While bringing it back to UNIT HQ, he swerved off the road to avoid hitting an Auton and was killed when his Land Rover crashed. Meg Seeley described the expression on his face when he died as "terrified". (TV: Spearhead from Space)