Sergeant Trott was a human who in 1982 hid inside a British nuclear bunker because he thought that there was a nuclear war. However, this was just an illusion created by giant cockroaches, who wanted the humans to stay in the bunker, so that they could use them as a food supply.
Trott was killed and eaten by the cockroaches. When the Eleventh Doctor arrived together with Amy Pond and Rory Williams, Colonel Bowe thought they had killed the sergeant. (AUDIO: Day of the Cockroach)