Invasion of the Earthmen The Avengers : Season 7 Episode 16

Following up a clue found on a dead agent Steed and Miss King pose as a married couple ,enabling them to infiltrate the Alpha Academy, where they claim they wish to enrol their 'son'. The academy is run by Brigadier Brett and is supposedly for youngsters who possess super-intelligence but it is in reality a training ground for a force who seek not global but extra-terrestrial domination.