Daniel, a weirdo writer who's fled his wife and home in Madrid, rocks up at an isolated country house occupied by Veronica, a weirdo hotelier and figurine-maker, and her invalid husband, name unknown and face unseen and voice unheard. A couple of couples rock up to the house to stay the night, only to be murdered by a black-gloved killer. When Elena, Daniel's wife, tracks him down (by methods unspecified), the truth finally outs.
Anyone who's read that and discounted a potential 'solution' based on its being too obvious should bear in mind that Spanish gialli typically were not possessed of labyrinthine whodunnit plots - oftentimes it seems that the identity of the killer was something of an afterthought (or, possibly more accurately, any attempt to obfuscate the identity of the killer was an afterthought). Paul Naschy's gialli exist somewhat outside this rule of thumb, but in a lot of cases (Night of the Scorpion being the best example that springs to mind) the films fail abjectly on a murder-mystery level. And this film's no exception-anyone who hasn't figured it all out after the first scene with Veronica's 'husband' (filmed, as all such scenes are, from a vantage point behind his chair, in which he apparently sits silently whilst somehow provoking his wife to continually reply to imagined slights) is, frankly, an idiot. (No offence.) Although maybe people who haven't seen Psycho are less likely to jump to the correct conclusion, who knows.
So - to deal first with the last - the 'twist' here is basically that there is no twist. This is possibly not how the filmmakers would see things - back in the late 1970s it's possible that cinemagoers wouldn't have seen Psycho (although at the same time they wouldn't all have been Fucking Idiots). I suppose the final few seconds of the film constitutes a twist of sorts, although it's up there with the least surprising reveals of all time (giving this film two entries on that list). Seriously - there are basically two potential suspects here, and it turns out that each of them is a killer. That'll catch out some people, but only because they've given the film too much credit and have sought a less obvious explanation.* There are multiple instances of that rare occurrence whereby the killer is presented as a red herring, rather than trying to smuggle them through proceedings in plain sight - probably partly due to the lack of potential killers, but possibly also because the filmmakers didn't really care, and were just ticking items (zoom in on black gloves - tick!) from a giallo checklist.
That's not to say that the film is badly-directed; it's not, and Leon Klimovsky does his usual solid job here, with a lot of slightly ropey handheld tracking shots to keep things visually interesting. There are also interesting touches such as Daniel's wife Elena being introduced to us through an extremely blue colour scheme. I mean, it doesn't necessarily mean anything, but it makes for a few nice-looking shots. The film clearly didn't have much of a budget, and was likely shot extremely quickly, but it's never really rushed or shoddy-looking (apart from possibly in the framing of the killer, as discussed in the footnote). The make-up effects are rudimentary-but-effective, with one sequence in particular perhaps revealing the true motives of the filmmakers.
I refer here to the scene in which young (very young) Irene Foster's nude body is slashed with a straight razor. Initially, the staging seems somewhat restrained - she grips the bedclothes around her when the killer starts hacking at her, and it seems that the violence will be limited to her facial area (very much a loose definition of 'somewhat restrained', I'll grant you). After she dies; however, the black-gloved hand and hairy wrist pops the bedclothes down, and gets to work on her breasts, in a sort of New York Ripper-lite sequence. This film was made shortly after General Franco's death, at a time when the Italian gialli were increasingly sex-infused (Play Motel, Giallo a Venezia etc). While the sleaze factor is never pushed as far as it was in the Italian flicks, it's certainly boundary-pushing for Spain, and I wouldn't be surprised if the giallo aspect was largely conceived of as a framework on which to hang the copious sex and nudity, with pretty much every character being nude at some point (apart from the young boy, who merely appears in tiny speedos). To be fair to the filmmakers, Daniel is played by Heinrich Starhemberg, the film's producer, and he doesn't exempt himself from the nudity requirements (much to no-one's pleasure).
The character of Daniel is a slightly odd one - he's the standard frustrated artist, but he's not exactly an obvious audience identification figure (SPOILERS) which very much lessens the impact of the climactic revelation regarding his character. It's difficult to know whether this was due to Starhemberg being an awkward and wooden actor, or if the role was intended to be played in this manner. If the latter, this decision falls squarely into the 'highlight a killer as a potential suspect' approach, as detailed above. (END SPOILERS) You'd have to say that the character as written is definitely a bit off, as even the most charismatic actor would have difficulty selling the sequence where Daniel strolls smiling towards a couple he's spotted copulating by the side of the road as Normal Behaviour.
The acting in general isn't too bad though, with the women being particularly impressive (and Antonio Mayans popping up for a brief period as well). Ágata Lys seemingly adopted a similar approach to Starhemberg, not really trying to sell Veronia as being 'normal', but instead giving a reasonably convincing portrayal of someone whose grip on reality is eroding at the seams. Much like Elena and her link to the colour blue, Veronica seems to be associated with wine being spilt, but I'll be goddamned if I know what that's supposed to symbolise.
The cinematography and music are solid, with sound design playing a more prominent role, at least initially, that it did in many gialli. The lack of attention typically shown for aural concerns was likely due to the practice of shooting without live sound, which presumably led to directors, who often weren't involved in the post-production process, seeing film as an almost-entirely visual medium. Here, though, we get a stormy night or two, and the mysterious husband in the upstairs bedroom provides an opportunity for some muffled bumps and smashes (not many though, and they gradually get phased out, presumably so the sounds aren't fresh enough in your mind that you ask "How did an empty chair make all that noise?" when the Big Reveal happens).
It's not a great giallo, or a great sex film. The filmmakers have somewhat stumbled on a slasher movie formula - disposable characters arrive at remote location and are murdered - but there's nothing here that wasn't done a lot better before 1978. And the odd bits where it seems like the filmmakers are at least trying to do something different (the blue and wine motifs, the hint at the strong bond between the young boy and Daniel) don't overly convince (the bond is quite convincing, but it's not something about which you want to really devote too much thinking time). But, it's not a bad film. It's short, competently made, and ticks pretty much all of the giallo boxes. Which just goes to show that there's a certain alchemy to filmmaking, a certain indescribable quality which separates the great from the good - this film is an attempt to construct a giallo from the sum of its parts. It succeeds, but great films have something extra, something indefinable, on top of that. But Leon Klimovsky, who retired after this film, didn't choose a bad aul note to exit on all the same.
*There is some ambiguity as to how many murders have actually been committed by one of the suspects - 'most, if not all, of them' would appear to be the answer if you examine the body of the killer in the set pieces (keep an eye on the wrist in particular), but it's likely that this is just slapdash blocking and editing, and the other suspect is probably intended to have committed all of the on-screen murders.