After a gauzy opening sequence depicting four shot-from-the-waist-down men killing a shot-all-over young man who was busy shagging a young woman on the beach, we jump ahead twenty years. A nine-strong party is sailing to a secluded island for their annual holiday-rich patriarch Uberto, his two sons and daughter, the significant others of all of the above, and Uberto's sister Elisabetta. We very quickly discover that the family are of the Ryan Giggs persuasion when it comes to respecting familial romantic boundaries, and we gradually discover that the rich, self-centred assholes at the centre of the film may be even worse specimens of humanity than we initially thought. Still, the good news is that they're all getting knocked off, one by one, by an unseen killer who purports to be Charlie, the murdered dude from the opening sequence.
While not a top tier giallo, this film does score points for a rigorous adherence to logic, only to immediately lose them for being somewhat overly predictable. The killer's identity shouldn't come as a surprise to anyone who's seen more than a handful of murder mysteries (SPOILERY hint: if someone is presumed dead, with no body found, it's likely that we haven't seen the last of them) or who's read the Christie source material, which hinges on a similar twist. The solution is so straightforward that one of the characters cracks the case (at least, claims to) early on, but chooses not to intervene because they're enjoying having their inheritance percentage bumped up with every bump off.
This leads to a schizophrenic climax, where the unmasked killer is being pursued by the survivor, which is standard procedure. The difference is, however, that the film's, and most of the audience's, sympathies lie firmly with the killer. The character who's cracked the case and chosen not to act has arguably been passively responsible for the final few murders (although you wouldn't want to extrapolate that logic to apply it to too many real world scenarios), and they then assume the mantle of the 'ultimate' killer (or, perhaps more accurately, they resume the mantle). They're certainly a less sympathetic and likeable character than the 'main' killer, who arguably turns out to be the most normal person in the entire film (discounting the boat's crew, who are summarily dispatched early on).
One obstacle to the audience's guessing the main killer's identity (SPOILERS AHEAD) is potentially a lazy mistake from the filmmakers, and potentially an intriguing hint towards a second killer operating throughout. The boathands are dispatched by both hands of a killer in a wetsuit, who we see escaping overboard after completing the act. Now, at the risk of offending Sofia Dionisio, I'm pretty certain that the person in the wetsuit is not her, but a male man. This is either poor body doubling/lax framing, or else we're genuinely meant to pick up on the visual cue and assume that the killer is a man. That might seem at first glance like a cheat, but Dionisio's Carla does mention that the first person she killed was XXX, a murder which occurred after that of the boathands. It's possible that she was discounting them as not being worth noting, given that they were merely collateral damage,. However, another explanation is that they were killed and the boat cast free by another family member, likely as part of an inheritance scheme. It doesn't really matter who did it, in a way, as every single potential male suspect is a killer, as is Carla.*
As I've stated, the end reveal isn't especially earth-shattering. We do get another twist, however, at around the hour mark. The opening gauzy murder, it turns out, was the four leading male characters murdering Aunt Elisabetta's lover Charlie, apparently for the crime of being poor. So, essentially, it turns out that the family (and in-laws) are all absolutely mental. There had been clues that they weren't all there-Venantino Venantini walking around in a pair of Daisy Dukes, and the fact that they chose to bury Charlie in a shallow grave right at the shoreline of the beach-but this revelation is the beginning of the process which ends with us cheering on the killer in the final showdown. The isolated setting works especially well as a cloak to mask the mass insanity, as interactions with the outside world would doubtless reveal the cracks before long.
There's a secondary twist/revelation which isn't foregrounded, or even remarked upon at all, but which is attendant to the reveal of the killer's backstory. Namely, Massimo Foschi has been shagging his cousin. Not the worst crime in a tale of multiple murders, but a bit icky nonetheless. Saying that, given the sexual proclivities of the family members, there's every chance that some cousin-shagging would've been willingly considered in any case. (SPOILEYS END)
To sum up, this is a solid, enjoyable film which pushes the envelope slightly with its pantheon of villainous characters, but which plays it safe and sound with regard to the plot. The nominal lead actor is killed off surprisingly early, which may have been an attempt at some Psycho-style machinations, or merely an indication that there was no clear 'star' among the nonetheless largely familiar cast. The cinematography and soundtrack are well above average, and extreme props to Mr Baldi for not even bothering to offer up a reason as to why no-one telephones for help. A visit from the mainland police is obviously never going to happen; we have nine guests, and nine guests only (once the boathands are killed) for the titular crime. We know that, Ferdinando knows that, so why waste valuable screentime offering a pat explanation for the lack of a phone call, when there are tits to be filmed?
*Another glimpse of a male killer comes during a strangulation scene. Although the snatch of a face somewhat resembles Massimo Foschi (and thus Cameron Mitchell), I'm fairly sure that in this instance the killer is meant to be Carla, and the fleeting glimpse is another error of framing.