Charlize Theron stars as Marlo in Tully, by director Jason Reitman. Picture: Kimberly French, Focus Features

Attention: Spoilers ahead.

"You pretty much are the baby right now," says Tully, the night nanny (MacKenzie Davis) for whom the recently released feature film is named. Impossibly young and thin, she has a surplus of energy as she plays opposite Charlize Theron's character Marlo, an exhausted 40-something woman who has just given birth to her third baby.

Marlo's much more substantial postpartum frame represents bodies seldom seen on film but recognized by most women: Bodies that have swollen and stretched and now sag; bodies that have a troubling tendency to leak blood and milk and other fluids; bodies that play host organism, to varying degrees, to other bodies.

As she coos over Marlo's newborn, Tully points out that the baby's cells are still circulating through Marlo's body, and will do so for years, an observation that, depending on one's frame of mind, can sound beautiful and intimately tender, or terrifying: An intrusion; a takeover. You pretty much are the baby right now. The baby has erased you.

Paradox is at the heart of motherhood, and of Tully, too: Is the mother an individual, or is she not? Is the woman who becomes a mother the same woman she was before, or is she different? (In a late scene, Tully points out to Marlo that virtually every cell in her body has been regenerated over the years, like a boat in which every board has been replaced. 

Is she, then, really the same person as she was 20 years ago? In what sense?) Is the woman whose body carries and feeds children all day the same woman who climbs astride her husband at night? Is motherhood beautiful and transcendent, or is it earthy, repulsive, and terrifying? Is the mother solid or liquid, a creature of land or a creature of the water? The film explores these questions with a frankness seldom seen in the mommy-Instagram scene.

The feminist philosopher Sara Ruddick has suggested that women's bodies, in the Western tradition, have stood for all that reason is not; for all that is opposed to rationality. Aristotle's identity principle, upon which all rationality rests, states that A is A, and A is not non-A. 

READ: Tully delves into the dark side of parenting

So perhaps it is not the whole truth to say of a pregnant woman that she is merely herself, and not an other, or to say that she is not-herself or not-baby. There is that mother-baby enmeshment at the cellular level, but also at the level of experience: What adult has not, in the full catastrophe of life with children, wondered something like where did "I" go?

Tully is disturbing, but not primarily because it appears to portray postpartum psychosis. That may be, though, notably, Marlo is not given a diagnosis at the film's end. 

Rather, it seems that mental illness is a vehicle for pointing to this complicated perception that, through motherhood, one's self is attenuated even as it is enlarged: You are not quite who you were, you are not quite the baby (but not quite detachable from the baby, either), and you are somehow not even quite who you are: 'You' will not always leak milk, blood, and water, and exist in a severely sleep deprived state, but for now, you are covered in milk in various stages of digestion and stumbling about in an adult diaper in the middle of the night.

Tully is disturbing because of its vivid evocation of just what a difficult and trippy road motherhood can be: The kind of experience that makes nearly everyone question her sanity and fitness for the task (and let out the occasional primal yell of rage). 

Another paradox, then, one that is perhaps unique to the modern, Western, middle-class mother: We manifestly cannot do it all, and feel damned when we cannot and damned if we need help. The twist, which I will not spoil, reveals this perfectly. 

We are expected not to need anyone, and to be everything to everyone: Educator, therapist, housekeeper, cook, lover, and more, including our own helper, comforter, and savior.