but it had some strange twists and turns and was ongoing in the u.k. as well as the u.s...,

wikipedia |  The extraterrestrial spacecraft can readily cross the vast distances between their planet and Earth at many times the speed of light, but are too small to carry more than one or two crew members. Their time on station is limited: UFOs can only survive for a couple of days in Earth's atmosphere before they heat up, deteriorate and finally explode. The alien craft can survive for far longer underwater; one episode, "Reflections in the Water", deals with the discovery of a secret undersea alien base, which shows one UFO flying straight out of an extinct volcano, which Straker describes as "a back door to the Atlantic". A special underwater version of the standard UFO design is seen in "Sub Smash". In flight they are surrounded by horizontally spinning vanes and emit a distinctive pulsing electronic whine that sounds like a Shoooe-Wheeeh!^[1] (This was produced by series composer Barry Gray, on an Ondes Martenot.) The craft is armed with a laser-type weapon, and conventional explosive warheads can destroy it. The personal arms of the aliens resemble shiny metal submachine guns; these have a lower rate of fire than those used by SHADO. Later episodes such as "The Cat with Ten Lives" show the aliens using other weapons, such as a small device that paralyses victims.

The show's concept was unusually dark for its time: the basic premise was that Earth had not simply been visited by extraterrestrial visitors, but indeed was under brutal alien attack, and that alien invaders were abducting humans to use as involuntary organ transplant donors. A later episode, "The Cat With Ten Lives," contains a sinister plot point which suggests that the UFO pilots are not humanoid aliens at all, but are in fact human abductees under the control of the alien intelligences, suggesting that, as in Captain Scarlet, the aliens, in the dialogue of Dr. Jackson, "may have no physical being at all and therefore need a container, a vehicle – our bodies".

The show also featured realistic, believable relationships between the human characters to a far greater extent than usual in a typical science fiction series, showing the clear influence of American programmes like The Twilight Zone and Star Trek and British action series such as Danger Man. One early episode, "Computer Affair," suggested an interracial romance between two continuing characters – something that was uncommon in British TV of the period – while others showed the heroes making mistakes with sometimes fatal consequences. Furthermore, relatively few episodes of the series actually had happy or (for the characters) satisfying endings.

The episode "Confetti Check A-OK" is almost entirely devoted to the breakdown of Straker's marriage under the strain of maintaining the secrecy of the classified nature of his duties. "A Question Of Priorities" takes this exploration further, and hinges on Straker having to make the life-or-death choice of whether to divert a SHADO aircraft to deliver life-saving medical supplies to his critically injured son, or allow the aircraft to continue on its mission to attempt a last-chance intercept against an incoming UFO. Two key images from "A Question Of Priorities" – Straker's son being struck down and his ex-wife declaring she never wants to see him again – are repeated in flashback in two subsequent episodes, "Sub Smash" and "Mindbender," suggesting that Straker remains haunted by these unresolved emotional issues.

Another episode, "The Square Triangle," centres on a woman and her lover who plan to murder her husband. When they accidentally kill an alien from a downed UFO instead, SHADO intervenes and doses the guilty pair with amnesia drugs. (This was decades ahead of a similar story device in Men in Black, and it was one that was deployed for similar reasons.) Straker realises, however, that the drugs will not affect their basic motivation and, worse, he cannot reveal the truth to local legal authorities. The end credits of this episode run over a scene set in the near future, showing the woman visiting her husband's grave and then walking away to meet her lover.