A potbellied loner spends his nights prowling around NYC looking for women to murder so he can scalp them and nail their hair on one of his many mannequins. That's pretty much the entire story, but what makes MANIAC unique is the genuinely creepy performance by Joe Spinell and the (almost) documentary style it's filmed.
I'm sure audiences back in 1980 were shocked by this gritty slasher, but I'd be willing to bet that first time viewers nowadays, especially after hearing all the hype around MANIAC's cult status, would be surprised at how boring it actually is. The one stand out scene (the shotgun scene) is fucking awesome, but other than that the rest of the film is kinda slow. Zero nudity, very little blood, very little gore, painfully low budget and although Spinell is creepy looking I'd be more worried about catching a disease from him than anything else.
Worth watching to see what all the hype is about, but don't get your hopes up too high. Double feature it with NEKROMANTIK for a night of 80's shockers that aren't really that shocking anymore.