"All things that live long are gradually so saturated with reason that their origin in unreason thereby becomes improbable" - Nietzsche, Daybreak
Under the Skin begins with a simple trope: human beings have something horrible inside of us.
The opening scene is a series of images of a light being obscured by a dark, phallic object. Shapes change, things enter one another, and we hear a voiceover of a woman learning phonetic basics. A sort of birth is taking place. The "protagonist" of Under the Skin is played by Scarlett Johansson. After she masters the basics of (English) human speech in this initial scene of slow, ominous reproduction, we are treated to the image of a brown eye, looking for all the world like a sphincter, contracting and dilating. This is prep work for the beginning of the film, where Johannson's character takes the clothes off of a woman in an utterly bare white room and puts them on herself. She's taking a step towards humanity in an otherwise empty vacuum by beginning to look like a functional human. A simple game of dress-up. When the woman is stripped bare, Johansson's character finds an ant on her naked body. In a Lynchian close-up, we see the ant's feelers and pincers in manic action. Without her clothes, the woman who Johansson is emulating through her clothes is unveiled, to the point where we find entropy working on her nude body. The ant, with its simple processes and machinic character, is a stand-in for the awkward unseen work of human development that underlies the clothes and the social character of the sort of functional human that Johansson is attempting to emulate, just as the close-up of the sphincteric eye highlighted the reflex behind the "window into the soul."
In Ghost in the Shell 2: Innocence, there is a poignant scene where a stand-in for cybernetic feminist Donna Haraway talks about the human urge to create life. She compares human children to dolls, implying that something unhuman has to take place in order for a certain sort of human normality to emerge.
Under the Skin uses a similar tactic. In order for Johansson's character to develop the human empathy that she develops later in the film, she first has to go through a set of animal/non-human transformations. The eeriness of the film comes from the fact that Johansson skips many of the steps to becoming human that we associate with the traditional process of growing up. She moves from being something other, being something utterly opaque to human values, to something that can operate in human society even though it does not possess the sort of inner life that we associate with fully autonomous persons. Even more disconcertingly, Johansson's character has near-complete control over a certain set of sexual human actions, the behaviors that are most often characterized as base or animal, that we don't associate with someone in the beginning stages of development. Playing on the trope of demonic children, Johansson's character is a sexual thing that behaves like an adult when it has to. Like a Barbie, the monstrosity of Johansson's character is that it is at once sexualized and infantile.
The psychosexual drama is readily available to anyone who has ever thought anything like Camille Paglia. Johansson, as cypher for chaotic nature, seduces men into a dark nowhere where they are sucked of their substance (quite literally). Since at least the myth of the Succubus, the idea of woman as gnostic temptress has been borne through the various transformations of the Western tradition. Women, as basically irrational and animal entities, seduce men to their darker instincts and then eradicate them.The feeling that we get from the shots while Johansson's character is driving through the streets trawling for men is that she is objectifying them in a manner that we typically associate with male predation, and this reversal itself is supposed to mark her as dangerous. Under the Skin attempts to undermine this trope through two strategies. First, Johansson's character is subordinated to another thing whose human body is identified as male. In this way, we're supposed to understand that not only are the men of the film objectifying Johansson's character, but that the process of objectification is also overseen by a male-identified entity. The chthonic seduction that Johansson apparently represents is caught up in a process run by a male identity, a process that is supposed to produce a product. This is hammered home when we see the ultimate fate of the men that Johansson seduces; their entrails are pushed out onto a conveyor belt that carries them into an infernal light. In this sense, Johansson's character is meant to be seen as simply another objectified part of a process of production, rather than an evil temptress who acts simply to abet the chaotic forces of nature.
The second strategy that Under the Skin takes to undermine its early premise is to show Johansson's character attempting to develop into a fully functional human. After reaching a very human sort of self-consciousness, not so subtly represented by her staring into a mirror at her human form, Johansson attempts to save one of the men that she has taken in. This man is horribly disfigured, and the idea of exploiting someone so piteous is supposed to trigger the latent empathy that Johansson's character has been developing throughout her time among us. She quickly goes on the lam into totally alien territory; she attempts to eat food only to choke it all up. She attempts a stuttering traditional relationship with a man only to throw him aside and run when she begins to understand what it is she's been seducing her targets with. The female body, it seems, has a hard time getting along in the world.
This difficulty with belonging reaches its apex at the ending of the film. It culminates in an attempted rape, and when the would-be-rapist discovers what's beneath her pretty exterior, he destroys her. The feminist undercurrent is nigh undeniable; women are only allowed to be human, and when they show signs of being something deeper and scarier than they first appear, men react violently.
This said, Under the Skin goes beyond a sort of humanist feminism to make a stronger claim. Rather than taking on the trope of men as imposers of rational order on irrational women, Under the Skin undermines another male strategy of making women into pure subjects. Men need women to be entirely surface, it suggests, because the idea that they are like men, that is, driven by something utterly alien to the status quo of rational sociality, is terrifying. The idea that despite their "soft" interior, the female libido is just as nasty and horrible as men's takes away the image of the feminine that men use in order to assume a position of authority over female naivete. Women are supposed to be fully human, penetrable, vulnerable, and transparent, so that men can be something more than human, deeper, more violent, and more animal. In this way, Under the Skin is a sort of critical rewriting of Darian Leader's take on Terminator; whereas the T-800 represents the emotionless and violent epitome of masculinity and renders the most masculine men feminine and vulnerable (i.e. the biker at the beginning of T2), Under the Skin suggests that there isn't an escape into the reductive innocence of feminine subjectivity. We can't simply acknowledge the fact that we are all vulnerable and socially interdependent persons. Johansson's character's attacker destroys her because she reveals herself to be more than an innocent subject for his defilement. Under the Skin turns our disgust with the otherness of female sexuality into a recognition of the perversion of a male sexuality that makes women into human subjects so that it can revel in the animal. Johansson's character is ultimately unacceptable not because she is an unfathomable force of nature or a fully autonomous person, but because she is both simultaneously. Under the Skin suggests that being a person is simply a part that we learn to play, and women are expected to inhabit the role perfectly or not at all.