An XV-104 test plane wanders into a UFO target area and the pilot, Paul Foster, makes sightings of both the UFO and Sky One. Angry and frustrated that no one believes his story, Foster suspects a cover-up and decides to conduct his own investigation into the activities of ex-USAF Colonel Ed Straker.

When an Alien Limpet UFO causes the destruction of Lunar Module 32, Straker demands a clean-up programme by the International Astrophysical Commission to remove hazardous space debris from Earth orbit. But Straker finds himself butting heads with I.A.C. chairman James Henderson, who wants a complete shut-down of SHADO operations...

"Important? What can be more important than your own son's life?" Straker faces the toughest decision of his career when his son is critically injured in an accident. The drug that can save his life can only be ferried from New York in time aboard a SHADO Transporter - the same Transporter that is in the prime position to track an Alien defector on the West Coast of Ireland.

A SHADO Technician, who is being blackmailed for information by an unknown party, is caught by Freeman, but not before the information is passed. The Technician, Paul Roper, divulges what information he gave to the blackmailers, to SHADO. After much work on the cryptic data, they discover it is part of a plan by the aliens to attack the Moonbase in a way that would be undetectable. Now it's up to Roper to redeem himself, as he is sent on the suicide mission to stop the attack. If he fails, the base will surely be destroyed.

Foster is abducted by aliens from a health spa and is being taken out into space, presumably back to the alien homeworld. However, the UFO was damaged and returns to the moon, crashing there. While the Moonbase personnel try to free Foster of the alien environmental suit, Foster wakes up in the steam room at the health spa. The episode begs explanation for how Foster dreamed in such orderly detail, including Carlin missing a shot and Straker angry at the missed shot, and Dr. Jackson's off-line comments to Straker before they go onto a visual link to Moonbase to guide them through removal of Foster's alien helmet.

After a pilot is killed when Moonbase operatives fail to give him course change information, the personnel are recalled to Earth for an investigation. A UFO crashes in northern Canada and the personnel are assigned to the operation to find the UFO. Gay Ellis is to coordinate the movement of the mobiles trying to approach the UFO.