The Faceless Ones

From http://www.bbc.co.uk/doctorwho/classic/episodeguide:

The TARDIS arrives on Earth in 1966 - on a runway at Gatwick airport. Polly witnesses a murder in a nearby hangar and is then kidnapped by the perpetrator, Spencer of Chameleon Tours. Subsequently Ben also vanishes. The Doctor and Jamie are left to try to convince the sceptical airport Commandant that there has been foul play.

It transpires that a great many other young people have also vanished, all of them while on Chameleon Tours holidays. With the help of Samantha Briggs, the sister of one of the missing youths, the Doctor and Jamie uncover a plot by the alien Chameleons to kidnap human youngsters in order to take their identities - for the Chameleons have lost their own in an accident on their home planet.

The Doctor offers to help the Chameleons find another solution to their problem and the kidnapped humans are released.

2 out of 85 (2.4%) raters say this story requires a previous story.