An intruder is a military aircraft and its crew that is tasked with penetrating deep into enemy air space, to disrupt operations.
The intruder concept dates from World War II, the first radar-equipped night and all-weather fighters were able to take advantage of the cover provided by night-time and meteorological conditions.
Once past the Chain Home radars, where they appeared to be returning bombers, they were free to attack RAF air bases.
The RAF eventually took up the same concept, using the Hawker Hurricane Mk IIc as a makeshift intruder in various theatres.
From late 1943, Bristol Beaufighters and de Havilland Mosquito intruders patrolled over occupied Europe, using Serrate radar detectors to hunt German night fighters.