One of my favorite types of movies (at least when pulled off successfully) is the "gearshift movie," a term coined by Paul Thomas Anderson. There's not always big twist, but an event occurs on-screen and after that the movie you thought you were watching becomes something completely different. Notable examples include From Dusk Till Dawn, which becomes a vampire movie partway through, and Anderson's own Boogie Nights, which starts as an entertaining look at the porn industry in the 1970s, then becomes a depressing drama after one character's suicide.
The shift in Gaspar Noé's Climax is one of the more dramatic in film history. Imagine if a movie started with the exuberant dance scene from Footloose, then the rest of the movie was Requiem for a Dream. Indeed, the film starts with a group dance rehearsal that's one of the most electrifying things I've seen on-screen in a long time. But what follows is a near real-time descent into drug-induced madness that will scar all these ambitious performers for the rest of their lives (in some cases literally).
What makes Climax such an impressive feat is how it makes you constantly feel whatever energy is present in each scene: The exhilaration of bodies in motion, dancing with abandon. The uncomfortable feeling that something's not right. The claustrophobia and anxiety of not knowing where you are. And then, sheer terror as your friends and colleagues turn violent. This is not a film for those with weak stomachs, not only because of the acts depicted on-screen, but also because of the handheld camerawork, which doesn't even stay upright for a good chunk of the film.
There's not much more I can say without venturing into spoiler territory, since it's best to go into the film blind. But I also have to note that if you're not a fan of Noé's stylish, provocative, "let me see if I can shock you" shtick, this one's not going to win you over. But if you're a fan of movies where you have absolutely no idea what will happen from one scene to the next, Climax is a trip you'll want to take.