I'd never even heard of this movie, which is probably ridiculous, and everyone else already knows about it. I only caught it today on some antenna-based movie station, which probably had it as a "horror" genre film in line with the coming Halloween holiday--which falls short of its meaning. But this tense, well-acted, psychological thriller starring Terence Stamp ("Kneel before Zod!) and Samantha Eggar actually is a brutal story about a Nice Guy who kidnaps a woman and basically works through so many dynamics of misogyny and abuse that it is practically a handbook of what fucks woman-blaming Manboys be.
This might not be a movie for everyone. It represents a stalker and kidnapper who has isolated a target and cut her off from everything, controlling her behavior--not always through violence, but sometimes through manipulation and even seeming pleasant at times. Such a stalker! He buys art books, makes tea, buys toiletries! He provides a place for his victim to stay and proclaims he's a gentleman and pretends he's above violence, but the drama between his need to control and her desire to be free is tense and real.
The title comes from his hobby of collecting butterflies. There is a deep scene where he shows his victim his collection, and she comes to the realization that she has been collected, like a thing to be had, dead, just like they are.
I'm not going to give away all that unfolds, but it is grim and sad and still and all, compelling and revealing. I'll admit I was rooting for a way for a happy ending to somehow come out of it.
Spoiler alert--no, watch the movie. The movie and the novel, by John Fowles, are available at Amazon. And probably elsewhere if you skulk about more than I did.