The Tardis materialises at Gatwick airport, and Ben and Polly are kidnapped by mysterious aliens. The Doctor, ably assisted by the resourceful Jamie, begins an investigation, while being hunted by both the aliens and the airport authorities.
He discovers that thousands of young people have been kidnapped by a tour operator, Chameleon Tours, and that someone on the airport has access to technology not known on Earth.
The six episodes fly by, as the plot twists and turns in a very entertaining way. There are some marvellous character roles (the Airport Commandant, the police officer investigating the disappearance of one of the missing youngsters, and the Chameleon leader), and of course the eccentric and lovable Pat Troughton, who's on top form in this story.
Sadly, it's Ben and Polly's last appearance in Doctor Who, so there's also a "will-they survive or won't-they?" aspect to their disappearance.
The story is - for once - not about monsters, but is a mystery; and the Doctor has to win a race against time before the aliens' plans can be completed. He isn't helped when the Chameleons start replacing his friends with doubles - as he never knows who will be the next to be replaced!
Some lovely acting, too, by Wanda Ventham, Colin Gordon, Donald Pickering, and Pauline Collins in some of the guest roles.
A classic Troughton story, with a surprise ending...