After being hired by a mysterious Japanese terrorist/freedom fighter organisation to steal some diamonds to fund their activities, mercenary Bruno has a change of heart, and takes the stones for himself. The only problem is that he's impregnated Mieko, the woman who hired him, who's also the sister of the group's leader. Not taking too kindly to Bruno's betrayal, the allegedly-pacifist Asians cobble together a ragtag band of other mercenaries, who are easily dispatched by Bruno when they track him down. Mieko comes closest to achieving her stated aim of killing him; she sprays him with bullets but he manages to get away, burying the diamonds on a hilltop before passing out. He comes to in a large country house, where the plot of Blue Eyes of the Broken Doll is reenacted, albeit with a slight twist ending...
It really is remarkable just how closely this film comes to resemble Blue Eyes (after a completely crazy opening, which plays as if you've crash-landed halfway through a film-in-progress). I always find it slightly reductive to list things that various films have in common, even if such methodology forms the basis for a large percentage of academic film studies writing, but the similarities here do bear repeating:
-Paul Naschy, on the lam, takes refuge in a large country house, where two sisters somehow find him irresistible
(SPOILER)-A third sister is confined to the house, and saddled with an apparent disability (END SPOILERS)
-Frequent allusions are made to the animalistic nature of humans (c.f. the title of this film), and pigs are slaughtered
-Naschy comes clean about his sordid past, which seems to act as an aphrodisiac to the sexy sisters
-A black gloved killer comes along late in the game, and starts offing peripheral characters
-After turning over a new leaf (/deciding that shagging sexy film stars is preferable to a life of crime), Naschy's past catches up with him, with tragic consequences
The giallo aspect burns brightly here for part of the running time, but come the finale it very much takes a back seat, to the point where it's not even made fully clear who the black gloved killer actually was. Not that it really matters, given the direction in which the film ultimately veers. Naschy was clearly a fan of the filone, with the black gloves present and correct, and extreme CU shots of eyeballs appearing at regular intervals (and the eagle-eyed among you, or the human-eyeds who pay attention, may see some clues as to the killer's likely identity by closely observing these images).
The gore is fairly tame, apart from one extraordinary sequence involving some hungry pigs which may have inspired a similar scene in Ridley Scott's adaptation of Hannibal. It forms part of Naschy's seemingly ongoing thesis which seeks to prove that humanity and animals have more in common that we'd like to think. The scene, with the pigs noisily and messily devouring a dead body, initially appals, but, given the repeated references to human beings consuming meat which season the film, it's clear that Naschy is seeking to hold a mirror to us. While also including some commercially-friendly gore, of course.
So, in most ways this film isn't really a proper giallo, although there is a mystery of sorts (albeit one of which you may not be aware of its existence until the Big Twist). You'll also find such filone staples as fascination-bordering-on-racism with exoticism, and sex and nudity. There are no real red herrings, but a case could be made for the entire plot strand featuring Mieko hunting Bruno to be a key component in a near film-long attempt at misdirection. This misdirection doesn't entirely work, as the proverbial pudding that is the central mystery is slightly over-egged through repetition of certain themes and lines of dialogue (although, as stated, it could also easily pass you by). Still, even if you guess the Big Twist before it happens, you should have a good time with this crazy little flick.