Spoilers for The Babadook below.
The Babadook is nominally an Australian horror film about a single mom (Amelia, played by Essie Davis) and her son (Samuel, played by Noah Wiseman) who are haunted by an evil figure from a children's book that comes to life with the intent to kill them.
The Babadook is actually about the damage clinical depression does to those who suffer from it: the toll it takes on their lives and their families. "The Babadook" isn't a phantom or a ghoul. It's a chemical imbalance.
To be sure, the movie plays this all relatively straight. We get all the horror tropes: the bumps in the night, the mysterious figures in scary garb, the croaky voices, the creepy kid who can see something we can't. If you wanted, you could read this as a rather simple—though expertly executed—horror flick. But the metaphor here is rather explicit: Amelia is suffering from a rather serious case of depression, one that puts her and her son alike at risk.
From a purely clinical perspective, Amelia seems to be suffering from a number of the symptoms of depression: Trouble sleeping, an inability to perform basic tasks such as driving, inability to control her negative thoughts, more irritability than usual. And as we learn in the film from her kindly next-door neighbor, "this time of the year" is hard for her, as it coincides with not only the birth of her son but the death of her husband. Sounds a bit like Seasonal Affective Disorder, no?
There're a couple of pages in the evil book that make all this rather clear. In them, we see the Babadook in its true form: as a spirit possessing Amelia. It's a shadow looming behind her, a presence that changes the way she behaves. These images are paired with the following lines:
I'll wager with you
I'll make you a bet
The more you deny
The stronger I get
You start to change
When I get in
The Babadook growing
Right under your skin
"The more you deny / The stronger I get" seems a clear reference to the fact that denying depression only allows it to took stronger hold. Eschewing treatment—whether out of ignorance or a desire to "tough it out"—is no way to get help. Indeed, it only worsens the problem.
Further, we're informed by the book that "You can't get rid of the Babadook." And, indeed, Amelia is unable to get rid of the Babadook. She traps it in the basement by film's end, feeding it a steady diet of worms to ward off its ravages. Again, thinking of the depression metaphor, the worms are a clear stand-in for Prozac or some other anti-depression drug. A depressive cannot get rid of their illness, but they can keep it under control with medical help.
Anyway, if you're into horror, The Babadook will probably appeal to you. Just keep in mind that you're witnessing something far more terrifying than a spook or a spirit.