Straker allows a UFO to land in Southern England in an attempt to capture the craft, and its alien crew, by sending out a fleet of SHADO mobiles. After the UFO lands safely, a lone alien survivor has a fight with a game warden and his dog, causing the alien's uniform to rip and his helmet to shatter. The alien kills the human with a machine gun-style weapon, drags the corpse to the UFO, and then self-destructs the craft in an attempt to get away by substituting the human with himself. Colonel Foster and his crew identify the charred corpse as the human and not the alien; moreover, they find a piece of the aliens uniform and use the surviving canine to track his location within the surrounding woodlands. Meantime, in a cottage close to the landing site, Liz Newton is conspiring with her lover Cass Fowler to murder Liz's husband, Jack. Liz plans to shoot Jack upon his arrival at home and then claim that she thought him to be an intruder. To that end Cass coaches her in the details of her alibi and it's made clear that he is the driving force in the plan. However, their scheme is foiled when the alien suddenly appears at the front door and is shot instead.

Foster and his investigation team arrive with the dog to find the dead alien, from which the conniving pair are taken to SHADO headquarters, where they are debriefed of the events, and eventually given an amnesia drug to make them forget everything that had occurred within the previous twelve hours.

Meanwhile Foster continues the investigation at the cottage. When Jack returns, Foster questions him to deduce the couple's murderous plan. At SHADO, Foster explains a theory that the pair originally planned to kill the husband and if released, there was no stopping them from doing it again. Straker points out that they cannot provide the body of an alien or turn over the pair to the authorities because SHADO is not in the "moralising business", from which Straker orders Foster to get everything possible about the destroy UFO, and Colonel Freeman to check on the autopsy of the alien. The pair are released. The final shot shows Liz in the near future looking at a non-identifying gravestone during the end credits with a far shot of an approaching male that appears to be Cass. RE:^[4]

• Ed Bishop — Commander Ed Straker
• George Sewell — Colonel Alec Freeman
• Michael Billington — Colonel Paul Foster
• Gabrielle Drake — Lieutenant Gay Ellis
• Dolores Mantez — Lieutenant Nina Barry
• Gary Myers — Captain Lew Waterman
• Keith Alexander — Lieutenant Keith Ford
• Ayshea — Lieutenant Ayshea Johnson
• Norma Ronald — Miss Ealand

Filming locations included Black Park and Black Park Cottage in Iver Heath, Buckinghamshire.^[1]

Review website anorakzone.com ranks "The Square Triangle" the third-best episode of UFO, calling it "a fine example of a relatively mundane episode being lifted into something far greater by a classic ending".^[5]

Describing it as "one of the most brilliant and successful" instalments, John Kenneth Muir praises the episode for its themes of sacrifice and "hypocrisy" in war ("Here, an innocent man must be allowed to die for the secrecy of [SHADO's] cause.") Muir also comments on the cinematography of the graveyard scene, noting that the camera zooms out to indicate "a kind of withdrawal, in shame and horror, at what has occurred".^[6]

• "The Square Triangle" at IMDb