The giant fly was the metamorphosed form of one of the giant maggots.
Despite its great size, it could fly. It attacked the Third Doctor and John Benton in Bessie, squirting poisonous slime at them. The Doctor threw his cape at it, causing the fly to become tangled in it. It was killed when it crashed to the ground. The Doctor bemoaned the fly's death, saying that it was "beautiful in its own way".