It is the dead of the British winter, but you would never know it if you were on the isle of Fara which is baking in an unseasonal heatwave. The locals are bemused, but one visitor seems to know more about the weather than he's letting on, Godfrey Hanson (Christopher Lee), who sets up a camera, mirror and sticks to craft an experiment. A tramp (Sydney Bromley) ventures to investigate but is left none the wiser, so retires to his cave to make his meal, but then he notices a sound rising in pitch which sends the elderly man into a state of terror, whereupon he meets a grisly fate. Meanwhile, another visitor, Angela Roberts (Jane Merrow), finds her sports car overheating and she has to stop by the roadside...
There was another island-based science fiction movie from British independent Planet which was released before Night of the Big Heat, and that was Island of Terror, a notably better effort than this, which it was seeking to emulate with the same director (Terence Fisher taking a break from Hammer) and many of the same cast, not to mention a similar alien invasion premise. If there was only one of these you felt the need to watch, then the first would be your better bet since this tended heavily towards soap opera theatrics in place of actual drama, which given it was set around a bunch of characters in imminent danger of being reduced to ashes you wouldn't have thought was particularly necessary, but they had to pad this out to feature length somehow.
The main subplot taking up so much screen time that it almost became the actual narrative if it hadn't been for the space aliens concerned Angela's endeavours to seduce the local pub landlord Jeff Callum, who also happens to be a successful writer retired from life in the big city to concentrate on penning his books. He was played by strapping British man of action, and man of prolific voiceover to match his lantern-jawed looks, Patrick Allen, not the most convincing of writers unless he was an Ernest Hemingway type in an earlier life, but you could well believe he would be a solid publican, especially in light of the manner he generously helps himself to his own supplies. In fact, it was a wonder that the cast were not paralytic by the end of the movie.
Yes, they certainly knocked back the beer, lager and whisky here, so much so that the plot might have been better off as a drinking contest between mankind and the invaders - we'd win hands down. The most terrible result of the threat may be the way innocent folks are turned to cinders, but when the bottles at the pub start exploding in the high temperature, it elevates events to a state of high tragedy, from the point of view of the characters at any rate. All a device to amp up the tension, which is noticeably lacking otherwise, mainly down to the business with Jeff trying to resist the charms of Angela and hide the fact they knew each other before on the mainland from his wife Frankie (Sarah Lawson), which has the minx being derided as a "slut" by Jeff, making excuses the attraction was purely physical.
Oh, that's all right then, you are forgiven, forward thinking and reasonable Jeff. Anyway, we have to forgive him as he is the first line of defence, though he has assistance from Hanson, not the Mmm-bop popsters but the scientist we saw at the beginning and briefly thereafter until Jeff boots down the door of his room in which he has been developing his photographs. The boffin informs the landlord about the reason for the heat, which is indicated for the benefit of us in the audience by having the cast slathered in glycerin and with wet shirts and blouses, supposedly drenched in sweat. Hanson claims people will be acting irrationally because of the too-warm conditions, which gets to the heart of the horror, not that people are being burned up, but that they lose control of their British reserve - Angela is nearly raped by Kenneth Cope! Peter Cushing is there too as the doctor who operates as a lamb to the slaughter when he gets caught up in the investigation, and the aliens themselves are belatedly underwhelming, though the reasserted British climate is the real hero. Music by Malcolm Lockyer.