The TARDIS materializes on the runway of Gatwick Airport. The time-travellers emerge and are almost hit by an oncoming plane. As Airport Security is coming for them, they scatter, and the TARDIS is hauled away. Before being united with the Doctor and Ben, Polly witnesses a murder committed by Spencer (Victor Winding). Investigating, the Doctor discovers that the murder weapon must have come from the future. Captain Blade (Donald Pickering) who ordered the murder, kidnaps Polly and removes the body. When the Doctor and Jamie arrive with the sceptical Airport Authorities, there is no evidence of a crime.
Spencer revives a faceless green alien and, connected to a machine, the alien takes over the body of an airport traffic controller. Meanwhile, Samantha Briggs (Pauline Collins) is searching for her missing brother. Investigating, they find fake postcards from trips that never happened. Ben finds Polly in suspended animation, then is grabbed himself and frozen. There are entire planeloads of missing tourists, who have been miniaturized. The plan is to seize 50,000 humans to replace aliens lost in an explosion on their planet.
The Doctor solves the mystery and frees the victims, and the nefarious aliens receive their comeuppance. Ben and Polly learn that it is 20 July 1966, when they first boarded the TARDIS, and they leave for home. The Doctor is left with Jamie, but the TARDIS has been stolen from Airport Storage.
Only Episodes 1 and 3 exist of the 6 parts. The BBC says the story will be completed with animation in 2020.
Part 1
Part 2
Part 3
Part 4
Part 5
Part 6