The film follows policeman Tony Saitta (Stuart Whitman)'s attempts to unravel the mystery surrounding his (way, way) younger sister's poisoning at a party, with various other guests who bore witness to her death also turning up dead. He unravels the mystery by tracking the movements of a black pearl necklace, in between indulging in fist-fights with transsexuals, loooong car chases with fences (humans who move stolen goods, rather than wooden railings) and eventually discovers that his sibling mightn't have been the angelic angel he believed her to be.
The film initially settles into pretty familiar giallo territory, albeit with the first murder being the subtle poisoning of a character at a party, rather than a black-gloved killer stalking and slashing. There's a rare(ish)-for-the-genre funeral scene, with a neatly-executed identity parade of all the potential suspects as they introduce themselves to Saitta and offer condolences. As Saitta begins to delve deeper into his sister's death (after insisting on exhuming her remains for an autopsy, which, amazingly, wasn't initially performed), all signs point to Martin Landau's doctor character as being the guilty party. He had begun an affair with the deceased, and was being blackmailed into continuing to see her against his will, (one suspects Woody Allen may have seen this film at some stage) and had attended her at the party as she died. Of course, given that Landau is the obvious suspect to viewers and police alike, he can't be the guilty party.
The investigation does progress initially with neat precision, and, unlike other gialli with police protagonists, it's very much driven by Saitta's raw emotions. His sister had attempted to contact him before her death, only for him to refuse the call in favour of continuing with an in-progress job (to be fair, I'd refuse a call from my sister if I was chasing a gang of bank robbers with tommy guns). This creates a brilliantly-conceived scenario whereby the only way for him to exorcise his guilt over his role in her death is by immersing himself in the very act, namely intense police work, which was the root cause and source of that same guilt. The script doesn't develop this concept very far, though, and Whitman's performance is the very definition of 'one-note'. It's as if he read the script, decided that the role called for an air of melancholy, then decided to eschew any attempts at nuance, or adapting that melancholia to the specific scene. Still, at least he's emoting at all, which is more than a lot of actors can manage.
The investigation, driven by guilt and love and delivered through the medium of melancholy, is again well conceived, but shakily executed. The deeper Saitta delves, the more he discovers about his sister Louise (and those around her, particularly her current and former paramours). It's pretty much universally negative discoveries, but, even so, she's not defined by one action or characteristic alone, as similar plot-catalysing characters usually are in gialli. As she's slowly fleshed out, and the subjective flashbacks take form, in front of our eyes, Louise emerges as a three-dimensional character (albeit three bad dimensions). As we strip away the plot layers, the characterisation layers build accordingly.
The investigative aspect is nonetheless shakily executed because Alberto de Martino hedges his bets, and incorporates sequences inspired by the then-burgeoning poliziotteschi genre, presumably to make the film more marketable (see the different titles, and posters, above). Unfortunately, unlike Sergio Martino's Suspicious Death of a Minor, there's no real attempt to gel the disparate elements, and the lengthy fight and chase sequences sit slightly at odds with the rest of the film. This is a particular shame because the stuntwork is very good, bordering on exceptional in the car chase. If the car chase involved two of the principals, or was better integrated into the plot, it would have worked even better. As it is, even though it involves Saitta chasing a newly-introduced character (just so he can ask him a question) who we know will play no real part in the film, it's still mightily impressive. It's just a shame that the investigative thrust is completely obliterated by the chase sequences, and never really recovers.
Technically, the film's a mixed bag. There's a huge amount of on-location shooting, and even some on-location sound, but de Martino wasn't a director who ever dazzled with his visuals, and he doesn't always take advantage of the Canadian cityscapes and countryside vistas. Similarly, the sound department, who seem to have been Canadian, must have been extremely inexperienced, in that they are far better at recording room tones than actors' voices (and they generate a lovely boom reflection on Saitta's car at one point).
Overall, the film's a mixed bag. It's far from top tier, even though with a little more care and skill it could well have been. Nonetheless, it's well worth checking out. It contains one the best assortments of acting talent ever assembled for a giallo, and it certainly contains the best stunts. And, where else can you see a famous lead actor having a kung-fu-off with a transvestite?? (Day of the Cobra, a few years later, that's where.)