A Naval vessel shoots down a UFO in the Atlantic, but the iralty rejects further investigation into the incident. Straker suspects a cover up and wants to know why, so Foster courts iral Sheringham's secretary, Sarah Bosanquet. What they discover could mean the end of all life on Earth...

"Unfortunately for them, an Alien came through that door instead of her husband." In the woods between Clare Cross and Lingbury, an injured Alien stumbles into a lonely cottage and is shot down by Liz Newton and her lover Cass Fowler. Liz and Cass are taken to SHADO HQ for questioning, but when Liz's husband arrives at the cottage, Paul Foster realises that the Alien's death was no accident!

Straker gets approval from above to build a probe with a camera that will track a UFO back to its home planet. Inexplicably, the Earth technology is capable of following a UFO travelling 8 or 9 times the speed of light and follows it home. The episode ends with a SHADO guy trying to convince Straker that they were wasting their time, even though they picked up some great pictures of the planet.

A UFO imbues three young people with incredible powers. One blows up a radar station, a second steals fingerprints and tries to get aboard a Skydiver sub, blowing it up. The third, a woman, tries to blow up SHADO HQ but the UFO that is feeding her power has now been destroyed.

Foster, whose radio is knocked out and his oxygen line cut, is presumed lost (but they did a crummy job, not even trying to find his body) during a confrontation with a UFO on the moon's surface. Foster is found by an alien, and together, they cross the lunar landscape, developing a friendship when the alien must try to do make-shift repairs on Foster's suit to keep him alive. At the end, a Moonmobile crew finds Foster and misunderstands his intent when he points them to the alien nearby.

After a UFO inexplicably destroys itself less than two miles from Moonbase, a mysterious crystal rock is recovered, and it causes the person in its vicinity to hallucinate. The man on Moonbase who found it thinks Mexican bandits have taken over Moonbase. It gets transferred to Straker's office, and he hallucinates that SHADO is simply a set for a TV series, he is an actor named Mr. Burns, Moonbase can be reached by stepping through two doors from the main control room at SHADO HQ, and "Mr. Straker" is not available. Straker breaks the hallucination by jumping in on the scene while the "character" Gen. Henderson is doing a close-up of the line they were doing when the hallucination began.

A SHADO Technician, who is being blackmailed for information by an unknown party, is caught by Freeman, but not before the information is ed. 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.

Reported missing, presumed dead, after a UFO incident which has left SID disabled, astronaut Craig Collins turns up alive and well. But as an operation to complete repair work on SID is planned, Colonel Lake and Colonel Grey discover that Collins is not the man he used to be...

Two of the SHADO people, dining at a fancy restaurant, begin to recall this event after one of them sees a diner who was part of Dalotek. A private company installed a facility on the moon which discovered an alien device that disrupted Communications between Moonbase and SHADO on Earth.

While picking up Lt. Lake at the airport, Straker and Lake find that time has been frozen except for them. Sawdust hangs in mid-air, Lt. Ellis is frozen mid-word on a viewscreen, and one man is taunting them, apparently working for the aliens.

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.

Foster is suspected of being a security leak, but during the trial, Straker and gang discover that Foster's apartment was bugged and shut down the disreputable business that is selling the spy gear.

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.

A flashback episode in which Straker recalls his marriage, followed by stormy days as he must work covertly to get SHADO up and running and recruit personnel. All this leaves his wife angry at his absence, suspicious at the things he might be doing (her father photographs Straker meeting a woman in a parking garage, a woman who will be working at SHADO). In the end, she gives birth to their son.

A UFO hides in a lake on a country estate of a well-known equestrian, and the equestrian himself vanishes. When SHADO tracks down the UFO and manages to destroy it in an exchange of pot-shots, the UFO breaks up and leaves a thing floating in the water. Treated as if it is a bomb, it is brought back to SHADO and examined, with unexpected results.

A mysterious dome is found under the sea, that at first looks like the Moon crashed into the ocean. Inside the dome, they find duplicates of SHADO HQ personnel, using non-alien technology - cassette tape recorders - to imitate SHADO instructions to Moonbase and Skydiver.

Freeman takes over temporarily while Straker deals with a woman apparently determined to investigate what SHADO is all about.

A woman awakens from a 10-year coma and begins telling the tale about her encounter with aliens, along with her boyfriend, while they were high on drugs. They toyed with the aliens, and the man apparently falls to his death from a bridge or building. The aliens drag him off. She has an accident and falls into the coma. She can't quite everything, but the old boyfriend is back, resuscitated by the aliens, and he drugs her so she tells him where the missing part of a device is. He activates what turns out to be a thermo-chemical bomb that will wipe England off the map when the four chemicals meet in a central chamber.