The Fly (1958)

Image copyright Twentieth Century Fox Film Corporation

Ostensibly a monster horror film, of the sort that was doing well in the box office at the time, this movie derives the majority of its thrills from the psychological effects caused by the transformation of the main characters husband into a human/fly chimera, and consequent decision to kill himself for the good of the world.  As a result, the titular monster spends a fairly small amount of time on screen, leaving a film that has more in common with a mystery thriller, rather that a strait horror fest.  Indeed, the New York Times film reviewer, Howard Thompson, described it as "...a quiet, uncluttered and even unpretentious picture, building up almost unbearable tension by simple suggestion."
The basic plot is one of a brilliant scientist, who accidentally discovers the secret of teleportation, and in the process of attempting to perfect it, has his head and left arm swapped with those of a house fly which has accidentally been teleported along with him.  Whilst this is often dismissed as logically flawed - why is the fly head human sized, instead of housefly sized; why is the exterior transformation instant, but the mental metamorphosis slow, the fly's instincts slowly asserting themselves - but this can be explained by thinking of the transformation not as a strait swapping of body parts, but rather a patterning error, of the type seen earlier on when a teleported ashtray emerges "back to front", with the writing on the back reversed.
Although the dialogue can feel dated in places, especially when dealing with attitudes to women, another important part of this film is the high quality of acting, with all the characters beautifully portrayed with a delicacy that makes them believable, with none of the overacting that mars many "monster horror" films.  As Brandt Sponseller says in his Classic-Horror.com review, "(Vincent Price)’s portrayal of the brother-in-law has just the right combination of emotions to capture a man who just lost his brother to a possibly insane sister-in-law who he loves as much as his brother and his nephew"
Overall, this film does not conform to "monster movie" stereotypes, with very few gratuitous "surprise shocks", relying instead on the beautifully realised portrayal of the psychological impact of the transformation on both the main character, Helena Delambre, and the victim, her husband Andre.  Although it starts out with an unusually gruesome (especially for the time) scene, where Andre is found crushed under a hydraulic press, and Helen confesses to killing him, as the film progresses we come to see that this was not an act of violence, rather an act of love; Andre having chosen to die this way rather than place his family in danger from the bestial instincts of the fly.

Having said that however, the film uses the few shock reveals to marvelous effect, hinting at the "grand unmasking", where the fly headed man is seen for the first time, a moment that even film4's otherwise fairly critcal review of the film describes as "...the most effective scene of its kind since The Phantom of the Opera in 1925.

image copyright Twentieth Century Fox Film Corporation