This a strange movie. It was directed by Anthony Margheretti who specialized in Italian Gothic horror films, but it was made at the height of "yellow (giallo) fever" in the early 1970's. As a result it is kind of a strange cross between a 60's Gothic horror movie and a 70's giallo. It is set in a 19th century Scottish castle rather than in a more modern-day Rome fashion house,for instance, but it has both a number and an animal in the title(the calling card of a giallo) and it features a series of nasty murders right from the opening credits when a man is butchered and his body fed to rats in the cellar. It also has an ape wandering around through secret passages in the castle, which doesn't fit either the giallo or the Gothic horror genre but might be some kind of homage to Edgar Allen Poe's "Murders in Rue Morgue". The story starts off when a young woman (Jane Birkin) returns home to her family's castle and is reunited with her neurotic mother, lecherous uncle, and angry young cousin (who owns the ape), and before long people start dying left and right. Oh yeah, there's also a cat wandering around who witnesses (some of) the murders, thus the title.
This was OK I guess, but I had a few problems with it. First, off they pretty much wasted the ape. If you're going to have an ape in your movie (or a man in an ape suit anyway) you should do more with him. Second, this is the only movie I've ever personally seen Jane Birkin in where she does NOT take her clothes off. As this made the movie significantly more boring I had a lot of time to ponder why this was and it finally occurred to me that she might have been pregnant with her daughter, the equally sexy actress Charlotte Gainsbourg, at the time. (And speaking of Gainsbourg, Serg Gainsbourg also has a cameo in this movie as a dubbed Scottish detective).
Finally, I was disappointed with the titular cat a little bit. It was kind of fluffy white cat like the one Blofeld is always stroking in James Bond movies. They should have used a more sinister black cat like the one named "Satan" that appeared in two gialli the previous year ("Gently Before She Dies" and "Crimes of the Black Cat"), but maybe he'd gone to kitty heaven or just refused to travel to Scotland (at least it wasn't a guy in a cat suit). Anyway, it seemed like they kind of forgot about the cat anyway towards the end. This was an alright movie, but it could have been better.
