Julia Sullivan is counting the days to her upcoming birthday, which coincidentally is also her insane twin sister Mary's birthday. Mary resides in a sanatorium (or a madhouse), virtually abandoned by Julia, who remains terrified of her sibling after a birthday ritual endured throughout her childhood. Her uncle, a priest who cares for Mary, reaches out to Julia to ask her to visit her twin, who has contracted a disease which disfigures her face to the point that her bone structure and even ethnicity seem to have altered. As the birthday approaches, Mary escapes from the madhouse, and, aided by a vicious rottweiler, begins dispatching people around Julia, including Sasha, a young deaf student of hers, and her friend Helen. When the big day rolls around Julia's boyfriend is conveniently/inconveniently called out of town, leaving her at the mercy of her Familia Loco...
The similarities to Happy Birthday to Me are manifold-the sibling angle, the countdown to a birthday (according to actress Edith Ivey, the shooting title was actually Happy Birthday), and even the dead dinner table(au) get a run out here. I'm not accusing anyone of plagiarism, but this seems to have been one of those occasions where there was something in the water which led to two near-identical projects being developed simultaneously (c.f. Deep Impact and Armageddon). Of course, it isn't impossible that a largely Italian crew, coming from a film industry which specialised in cheap and fast knock-offs, could have churned out a Happy Birthday to Me facsimile in double quick time, but I'm not actually suggesting that this film copied HBtM, or vice-versa (I'm genuinely not, swearsies).
There's not much of a central mystery here, with the killer-whoever or whatever they are for any given scene-always being clearly identifiable. There is a twist around the hour mark, when we discover that Mary (and the dog) aren't the only killers in town (but you'd probably guessed that [SPOILERS!] as soon as you read the word 'priest' in the synopsis). END SPOILERS! There are regular set pieces throughout the film, most of which are staged and shot very effectively, with one in particular standing out from the rest. Running around ten minutes in length, it sees Julia's landlady (the aforementioned Edith Ivey) engaged in a fascinating game of cat-and-mouse with the killer. And it really is cat-and-mouse (not literally, the cat's already been murdered and there are no visible mice in the film)-I've never seen a sequence in which the killer toys with the victim quite as much as we see here. It becomes almost like a dark piece of performance art, with the chase conducted at such a leisurely and relaxed pace that it seems that Assonitis is seeking to define and then tread (and tread leisurely) the line which separates tension from tedium. And, to be fair to the guy, he's largely successful, aided in no small part by the gleefully over-the-top acting.
The central location, which becomes a madhouse by the end of the film, had previously been used for Lucio Fulci's City of the Living Dead. It's an incredible building, in that it seems to contain about seventeen flights of stairs (and we get to see them all during the cat-and-mousing). Its windows also seem to be unopenable, and made of some unbreakable material, as none of the people who are terrorised there even consider them as an escape option-they're card-carrying aficionados of the 'go up the stairs' method of psycho evasion. Julia's friend Helen even chooses to await death when her shirt gets caught on the bannisters rather than divest herself of an item of clothing-if only she was as free and easy with her clothing as the teens populating the other slashers of the day.
Animal lovers should be forewarned that there are a couple of (staged) mammalian deaths, one of which is pretty prolonged and brutal. A rottweiler also sports a ridiculous snarl on his face which was presumably achieved through the use of some sort of mould/clamp fitted under its lips. The initial effect is fairly creepy-albeit there is a striking resemblance to Homer in that episode where he's being forced to smile through the use of hooks-but the final shot of the angry dog seemed to suggest a sadness in the eyes, (though I may be projecting). Speaking of animals in films, why is it that cats always meow loudly right as they're leaping into the frame?
And speaking of animal deaths in Italian films we come to the score, by the great Riz Ortolani. Taken on its own merits it's a very effective collection of cues; however, anyone who's familiar with the Cannibal Holocaust soundtrack won't be able to escape the undeniable similarities between the two scores, with the Madhouse one ultimately sounding like an extended homage to his best-known work. But I don't want to seem like I'm complaining-it took me out of the film, but the Cannibal Holocaust score is amazing, and I'm always happy to be reminded of it. And plenty of Ennio Morricone and Bruno Nicolai scores are self-referential also, so play on, Riz.
All in all, this isn't a top tier giallo (it's probably not an any-tier giallo, but no matter). It's solid entertainment made by solid pros, with better-than-solid acting and cinematography which raise it above many of its competitors (although Happy Birthday to Me, its closest competitor, is actually a pretty slick production too). There are some underwhelming moments-the first reveal of Mary is pretty botched, with her hair swishing over her face for most of the time she's on screen-and the bizarre approach the main killer adopts when menacing their victims means that the climax isn't quite as chilling and hysterical is it probably should have been, but it's worth a watch-if for nothing else than the batshit scene of Julia's uncle using the medium of a public church sermon to deliver a highly specific message to her about her duties as a sister. Oh, and said sister also does some of the best 'limp body' acting you'll ever see at the film's climax. And if those teases don't make you want to rush out and see this, I don't know what would.