The film opens with a double murder, that of a wheelchair-bound countess by her husband (staged to seem like a suicide), and that of the husband by an unseen assailant. The murdered couple lived in a mansion overlooking a secluded bay, which has been the subject of a bid from a property developer. A group of four horny teens (/young adults) arrive at the mansion for some frolicking, skinny dipping, dancing and shagging, and are dispatched one by one by two. The countess' husband's daughter (stay with me) arrives with her husband. They claim to be looking for her missing father, but are really seeking to secure the wife's inheritance. She discovers that the countess had a secret illegitimate son, who lived not-so-secretly in a shed on the bay, and both sides try to murder their way to the inheritance.
That final line is and isn't a spoiler; the motive behind most of the murders is never really in question. The identity of the murderer of the countess' husband and the teens is initially concealed from us, but the reveal isn't really treated as a climactic moment, as it would be in other gialli. This is mostly because that murderer is only one of several, all of whom are working to get their grubby (and bloody) hands on the titular bay (of blood).
Despite being set in the countryside, Bava doesn't really push the 'exotic other' angle in the same way that other gialli (eg Don't Torture a Duckling and The House with Windows that Laughed) with rustic locations often do. The residents of the bay are a mixed bag, with the earthy 'charms' of Simon, the illegitimate son, contrasting with the insect-studying Paolo. Paolo's tarot card-reading wife, Anna, does represent a form of exotic mysticism, but Bava's films often incorporate supernatural touches matter-of-factly (eg Baron Blood and Hatchet for the Honeymoon). There's no real sense of Anna's clairvoyance, if indeed it's real, being tied in to her association with, and residence within, nature.
Instead, the countryside, specifically the bay, is merely an asset to be fought over. Paolo, and to a lesser extent Anna, makes the case for its preservation on ecological grounds (indeed, one of the film's many, many titles was Ecology of Crime), a plea which falls upon the deafest of ears. You get the sense that Bava was on the side of the conservationists, and was satirising an innate greed which characterises almost all of the protagonists. Even Paolo and Anna aren't exactly sympathetic characters, constantly bickering and extraordinarily ineffectual at winning anyone over by their arguments. This is another of the film's themes-the ability to influence others with power and money.
As stated, there are several 'murderers' in the film. There are essentially two factions warring over the inheritance rights, with the two 'puppet masters' each largely inducing others to do their murderous bidding. Thus, we see the corruptibility of man writ large on the screen, with the promise of financial reward proving far more important than ecological conservation (cf the world right now). The teens are perhaps the only characters ostensibly free from the vices of greed and selfishness (Paolo and Anna are definitely selfish), but even there we can witness the seeds of corruption being sewn (or in full bloom, depending on your mores).
The French girl, Laura, insists on both of the guys accompanying her to party in the apparently-abandoned house, refusing to allow either of them to accompany Brunhilde on her skinny dip. This is selfishness and greed in operation on a physical, rather than financial, level. And, depending on your world view, this skinny dipping, as well as the shagging Laura and Filippo engage in, represents corruption of a sort. Bava was a religious man, so I'd be curious to know whether or not he viewed the teens' murders as being a punishment for their transgressions (they did break into two properties, which also represents a transgression, although one for which death seems an over-the-top response). Either way, the sex=death cliché is definitely in evidence here in nascent form.
This slasher section also features some excellent quasi-POV shots, which recall those from the climax of Blood and Black Lace. The camera drifts towards the victims, inexorably honing in on the prey with a single-mindedness and clarity of purpose which seems otherworldly. It's possibly a representation of the murderous lust which drives the killer; their id manifested through the camera movements, rather than an accurate attempt to portray their literal point of view.
The film doesn't quite recapture the heights of this early stalk-and-slash sequence, but it's always deliciously entertaining, and it exhibits a frankly breathtaking disregard for the sanctity of human life. The mystery at the centre of the plot is perfunctory at best, with the message-greed is bad-being far more important. One detail, involving the repurposing of a despairing diary entry is ingenious, and it's a pity it's wasted in a plot that doesn't really gain any from its inclusion. Speaking of ingenious, the film is an excellent example of how to stretch a minimal budget to the absolute maximum. Bava famously used a child's red wagon as a dolly, and carefully-selected camera angles maximised the locations. Comparing the film's look and style to an earlier effort like Black Sunday shows just how versatile and skilled he was; Black Sunday's lighting effects and camera movements have been replaced here by an extreme economy of filming (and money), with zooms and pans the order of the day.
This was Mario Bava's final giallo, and he clearly intended to stretch the traditional greed/inheritance plot to breaking point and beyond as a way of saying goodbye (his previous two gialli definitely suggested that he was jaded with the filone). The new, Argento-influenced films which prioritised insane killers and raw sexuality weren't for him (we come back to the question of how he really felt about those horny teens), and it's fitting that the final scene is one of his most playful moments. Bava's work always had an underlying sense of humour about itself, and the scene in question seems to work as a big wink at the audience, reminding them that we've just been watching a fun movie, nothing more (reminiscent of the end of Black Sabbath). If you're a fiend for subtext, though, you could view the scene as an extension of the theme of the corruptibility of man, as the final murderers have clearly learned how to use their weapon by observing and copying others. And so, the chain reaction (another title of the film) of violence begat by greed continues on to infect another generation.